Eli5 me "end game"

Hey guys - old vanilla wow player here just getting back into it in Retail.

I've been looking around googling and some probably pretty obvious answers I just can't seem to find - can someone help me out and explain (or shoot me a link)? Here we go:

• Dungeons: what are these tiers (heroic?)? How do I "get there" ... literally ?
• Raids: what are these tiers (lfr?) how do I get there literally (I.e., do I walk to black wing lair?)
• keys? No one knows ..
• old "legacy" raid / dungeons - that one shot stuff - how do I get there? Is it just walking to molten core and going at it ?

I also don't understand how they messed with pvp / Pve gear ... but that's for another day.

Would greatly appreciate your help for a now noob !