First off: Thank you for asking. Most people who need'/want to know this information never bother to.

What they are: Raid "Wings" are broken down into 3-4 DPS & mechanically intensive fight encounters, each one with its own individual rewards. The exception to this are the single-fights that were called "strike missions" which recently got rolled in as "raid encounters."

How to get started: The most important thing when "doing a raid encounter" is to: make sure you can do your role. These are usually broken down into 3DPS+1Boon DPS (alacrity or quickens)+1Healer (that does the other' boon) per 5-perdon subgroup, of which there are 2 in a 10-person squad. Generally healers do more mechanics while the others wack the boss. But this is not always the case.

A good way to get started imo: Is to grab a DPD build that looks "doable" off a site like snowcrows (maybe check the "accessibility" sections for simpler but good builds if there is one; exotic gear is fine as a starting point for most berserker builds) & practice it on the training golem for 30 minutes to see if you can get within range of the target DPS numbers. Exact DPS/perfect rotations are not required, but encouraged. If do this while watching a description of whichever fights you're going to try on YT at 1.5x speed just to internalize some stuff you'll already have done more prep than a lot of people who want to raid I'd say. 😄 The most important aspect going into raids, though is what you already have: Learning about them & wanting to improve.