This is my general assumption. But ultimately just use whatever you think suits.
Like: When you like the post or just want to kudos the OP, the default 'This is cool!' button
Dislike: When you dislike a post and want to just be a twat about it.
Agree: When you like the post so much you want to show it by a nice green tick!
Disagree: When you disagree with a post enough to put a big X underneath their post
Achievement: I don't know what the fuck this is. I can't use it so it doesn't matter.
Winner: When a post goes above and beyond, can also be used ironically
Semper Fidelis: "THE W.H.O FEARS THE SAMURAI!" - Or anything militaristic or warlike I guess? OORAH!
Deviant: Never used it, probably has sexual connotations so I feel wary of using it.
Informative: When you think a post is a genuinely informative read and you want to make this known.
Thunk-Provoking: The ironic version of Informative.
Feels: Women usually use this one.
Islamic Content: "Inshallah!" - I see this one being used ironically, strangely interchangable with Horrifying. It's probably also used a lot any time a Muslim sporadically blows up and gets on the news.
DRINK!: I'll drink to this post!
Lunacy: I don't see this one used a lot, probably interchangeable with dumb and autistic
Autistic: A classic, used to call someone autistic or just used to react to some autistic person of interest.
Horrifying: Any time someone posts a news article from VICE or shows people the inner workings of the furry community, also just stuff that makes you cringe. Usually followed by demands for you to spoiler whatever you posted.
Optimistic: Dark times ahead? Is the world on fire? Well that one post designed to bolster the morale of those who read it is an optimistic post, also gay stuff because it is a rainbow.
TMI: Never see this posted either, FUCK SAKE NOW I HAVE! probably used when someone reveals their powerlevel or shares something too intimate.
Late: Any time I post any news about anything whatsoever on this forum.
Dumb: Interchangeable with autistic and Lunacy to a lesser degree, when someone says something retarded, if you get this reaction, you'll probably get a good few more before the post sinks into obscurity
Mad at the Internet: Reserved for niggas who just can't keep themselves from bitching about stuff online.