The Halal is that which is allowed, that from which the burden has been released (inhallat) and the Lawgiver - who is Allah Most High alone - has permitted it.
The Haram is that which the Lawgiver has prohibited in an absolute way such that anyone who transgresses this prohibition exposes him/herself to the possibility of Allah's punishment in the hereafter and possibly to a legal punishment in this life as well.
Beguiling us into consuming, doing and speaking the haram is one of the major objectives of Shaitaan. If he can't get us to commit shirk or to reject faith altogether - thereby becoming one of his permanent companions in the eternal fires of hell - he can at least try to get us to commit more bad deeds than good or to commit good deeds which are not accepted in order that he at least get some temporary company in his torment of the hereafter. This is why Shaitaan is our biggest and most dangerous enemy. (And that is why in many of our affairs, WE are our own worst enemy once Shaitaan has gained a foothold.) Allah said:
{Yaa ayyuhaa an-naasu kuloo mimma fiy al-ardhi halalan tayyiban wa laa tattabi'oo khutuwaat ash-shaitaan innahu lakum 'aduwun mubeen.}
{O, people, eat from what is on the Earth lawful and good and do not follow the footsteps of Shaitaan. Surely, he is to you all a clear enemy.} Al-Baqarah: 168
The Din of Allah - which culminated and was completed with the message of Muhammad (sas) - has directed us to every wholesome thing and made it halal and has warned us about every harmful thing and made it haram. The Din of Allah is perfect and no one has the right to legislate anything - especially in the area of halal and haram - ANYTHING once Allah has legislated an issue. Those who issue legislation which is at odds with Allah's legislation are in the same category as shaitaan himself and fall under the same name of taaghoot. Taaghoot means those who are worshipped beside or along with Allah and they accept that and/or call the people to it. The Prophet(sas) explained in the hadith of Suhail Ar-Rumi that when the Christian leaders made things lawful and other things unlawful without authority from Allah and called people to follow them, that made them objects of worship and to follow them in that was to worship them.
Because of this, to change Allah's law in any way - whether ot make the unlawful lawful or to make the lawful unlawful is all part of Shaitaan's agenda and is a major sin indeed or perhaps outright kufr. Allah said:
{Yaa ayyuhaa alladhina aamanoo laa tuharrimoo tayyibaati maa ahalla Allahu lakum wa laa ta'tadoo. Inna Allaha laa yuhibbu al-mu'tadeen. Wa kuloo mimma razaqakum Allahu halalan tayyiban wa ttaqoo Allaha alladhiy antum bihi mu'minoon.}
{O, you who believe, do not prohibit the good things which Allah has made lawful for you and do not transgress. Surely, Allah does not love the transgressors. And eat from that which Allah has bestowed on you - lawful and good - and beware of Allah the One in whom you believe.} Al-Ma'idah: 77-78
Believers are to partake of the good and the lawful which Allah has granted us. The halal is rightfully ours in this world. The disbelievers get it in this life, but they have no right to it. We should not fall into the falsehood of the Christians and others who believe in forgoing halal things and hope for them in the hereafer. A believer should enjoy Allah's good things in this life AND in the hereafter. It is the haram which we must forgo in this life and order to preserve our share in the hereafter. The Muslim who drinks alcohol in this life will have no share in the perfect wine of Paradise. The disbeliever may partake of both the halal and the haram in this life, but they have no share whatsoever in the hereafter. Allah said:
{Qul man harrama zeenata Allahi allatiy akhraja li 'ibaadihi wa at-tayyibaati min ar-rizq. Qul hiya lilladheena aamanoo fiy al-hayaati ad-dunya, khaalisatan yaum al-Qiyama. Kadhalika nufassilu al-aayaati liqaumin ya'lamoon. Qul innamaa harrama rabbiy al-fawaahisha maa dhahara minhaa wa maa batana wa al-ithma wa al-baghya bi ghairi al-haqqi wa an tushrikoo billahi maa lam yunazzil bihi sultaanan wa an taquloo 'alaa Allahi maa laa ta'lamoon.}
{Say: who has prohibited the Allah's beauty which he brought forth for his slaves and the wholesome provisions? Say: they are for those who believe in the life of this world, exclusively on the day of resurrection. Thus, we clarify our signs to a people who know. Say: my Lord has only forbidden obscenities - both the apparent and the hidden - and sin and transgression without right, that you should associate others with Allah though He has sent down no authority for that and that you should attribute to Allah that of which you have no knowledge.} Al-A'raaf: 32-33
Manipulating what is lawful and what is prohibited at odds with the law which Allah sent down is one of the forms of attributing to Allah that of which you have no knowledge. Allah said:
{Qul ara'aitum maa anzala Allahu lakum min rizq fa ja'altum minhu haraman wa halalan? Qul a Allahu adhina lakum am 'alaa Allahi taftaroon?}
{Say: Do you see that provision which Allah has sent down for you and of which you have defined prohibited and lawful? Say: Did Allah grant you permission or do you concoct a forgery and attribute it to Allah?} Yunus: 59
So, while partaking off the halal and not neglecting our share in this life, a believer must also beware of the haram, lest he fall into some of it. This involves putting some distance between us and the haram. The Prophet(sas) warned us about this in the following hadith from Muslim and Bukhari:
"Inna al-halala bayyin wa al-harama bayyin wa bainahumaa mushtabihaat laa ya'lamuhunna kathirun min an-naas. Faman ittaqaa ash-shubuhaati faqad istabra'a lideenihi wa 'irdhihi. Wa man waqa'a fiy ash-shubuhaati waqa'a fiy al-harami ka ar-raa'iy yar'aa haula al-himaa yooshaku an yarta'a fihi. Alaa inna likulli malikin himaa. Alaa inna himaa Allahi mahaarimuhu. Alaa inna fiy al-jasadi mudhghatan idhaa saluhat, saluha al-jasadu kulluhu, wa idhaa fasadat, fasada al-jasadu kulluhu. Alaa wa hiya al-qalb."
"The lawful is clear and the prohibited is clear and between them are some unclear issues which most of the people do not understand. So, whoever steers clear of these unclear issues has obtained innocence for his deen and his self-respect. And, whoever enters into the unclear issues, enters into the prohibited just like the shepherd who grazes his sheep around the royal pasture always about to graze on it. Surely, every sovereign has a forbidden zone. Surely, the forbidden zone of Allah is all that He has made haram. Surely, in the body there is a small chunk of meat. If it is wholesome, the entire body is wholesome and if it is corrupt, the whole body is corrupt. Surely, it is the heart." Muslim & Bukhari
Avoiding the haram is imperative on every Muslim and must not be taken lightly. The Prophet (sas) warned us that the Ummah would reach a state when the haram gets taken lightly. He (sas) said:
"Ya'tiy 'alaa an-naasi zaman laa yubaali al-mar'u maa akhadha: a min al-halali am min al-haram?"
"A time will come over the people when an individual does not worry about what he takes: is it from the lawful or from the prohibited?" Al-Bukhari & An-Nasaa'i
Committing the haram jeopardizes our other actions and places them in danger of not being accepted by Allah Most High. The Prophet (sas) mentioned the traveller on a long journey (usually one whose du'a is quickly answered by Allah), but "whose food is haram and whose drink is haram and whose clothing is haram and who has nurtured himself on haram - so HOW could his du'a ever be answered?" Muslim & At-Tirmidhi
Sufyan Ath-Thawri said: "To spend of the haram in an act of obedience to Allah is like trying to clean soiled clothing with urine. Soiled is not cleaned except with water and sin is not removed except with the halal.