but in general:
ElvUI (core) - The core addon, similar to having 100000 addons bundled into one, but taking up less resources
ElvUI_AddOnSkins - don't you hate it how some addons still look like they're being skinned by blizzard? this fixes it and makes them look like ElvUI
ElvUI_Enhanced - tons of little tweaks which make your life easier
ElvUI_VisualProcs - do your played classes have any procs? this will show procs visually and play a sound when your [proc] procs!
ElvUI_LocPlus - adds a new info bar with location and two new info panels to the top middle of your screen (tho you can move it around)
ElvUI_RaidMarkers - adds a small new bar with raid marker control buttons on it, so you can target and click to apply them
ElvUI_ExtraActionBars - your character actually has about 10 or 12 action bars, tho some are used for shapeshifts and stances, this lets you get to all of them
ElvUI_MicrobarEnhancement - will allow you to visually change the microbar (that thing with character panel button, achievements button, talents etc)
ElvUI_SwingBar - adds a "cast bar" for weapon swings (useful for melees)
ElvUI_CastBarOverlay - will force all castbars to be moved and scaled into the unit frame of the person casting. You'll either love it or hate it
ElvUI_DTBars2 - will let you add new "panels" onto your screen, which you can resize and add information to, or simply use them as a backdrop for an external chat panel
ElvUI_EnhancedFriendsList - has certain tweaks for the Friends List to make it visually prettier and to have some data about the person beyond "unknown" for offlines
ElvUI_CustomTweaks - has a few extra tweaks for things you're less likely to want to tweak
ElvUI_CustomTags - has extra tags for unitframes if you want to mess around with filters(?)
ElvUI_AuraBarsMovers - lets you move the AuraBars to different locations if you wish (AuraBars are visual "channeling" style bars which show duration of your dots and debuffs on the target)
ElvUI_BagControl - lets you tweak some parameters about when your bags automatically open / close and some other things
ElvUI_ChannelAlerts - will let you play a sound each time you get a message in /g /o or any "numbered" chat (like whispers, but everywhere you want it)
ElvUI_DataTextColors - will change text color for some DataTexts to make them pop out more
I've roughly grouped them up into:
1. you just about need these
2. you should consider these
3. these are situational but can come in handy
4. these are minor, only get if you're serious about tweaking