Мужчина — это лицо мужского пола после определённого возраста, часто после достижения совершеннолетия. Слово "мужик" может использоваться для обозначения мужчин низкого культурного или социального уровня и как правило считается если не оскорбительным, то уничижительным. Никого из известных благородных героев, таких как д'Артаньян, мужиком не называют. Это слово также употребляется в русской классической литературе применительно к крестьянам мужского пола. Женская форма этого слова — "баба". Однако в воровской и криминальной среде, и вообще любой среде, где ценятся не хорошие манеры или голова, а грубая сила, это слово часто употребляется как позитивное. Если там хотят кого-то похвалить, то можно услышать выражение "настоящий мужик".

"Мужчина" means a male person who has achieved full legal age, usually 18 years. The word "мужик" could be used to describe individuals of low cultural and/or social background. The word is considered to be disparaging, if not insulting. You don't want to describe popular noble heroes, such as d'Artagnan, with this word. The word was also used in the Russian classics to refer to male peasants. Female peasants were described as "баба". In the criminal milieu and by extension in any environment in which brawn rather than intelligence or fine manners is admired, the word is often used with a rather positive connotation. "Настоящий мужик" could be a form of praise there.

I like the second explanation but some will go for the first one. I would just add. For some reason in Russian we have masculine words that end with consonant sounds... This is true for all but the words daddy, uncle, grandpa and man. They are папа, дядя, дедушка и мужчина. The word мужик ends with consonant... for a change just like all other words with masculine gender in Russian. ) There is a saying that мужик is someone, who drinks vodka, uses cursing and beats his woman. And some low class man believe that is true. In the same time if anyone would try to prove their masculinity to a woman he would do the opposite things of corse. The opposite side of this word is a handyman and someone who can protect and care for his family. So when people say "ты настоящий мужик" they may mean both meanings depending on the context. However, мужик is a very Russian word i think with spoiled meaning. Мужчина is more civilized word.

Hello. it`s absolutely the same, exchangeable. Мужчина - it is formal.( you can find it everywhere:newspapers, formal conversations, demands etc.) Мужик - it`s informal. We use it more than the first one in our daily speech, sometimes it can sound rude, but it depends on context and situation.

"Мужчина" is the standard form. "Мужик" is very informal, colloquial and casual.