Classes are almost a nebulous concept in GW I feel. Due to how profession system works, you can mix and match a primary profession (which you choose at character creation and can't change) and your secondary profession (in which you have the ability to change). So you could have an Elementalist/Mesmer setup or a Elementalist/Ranger or something even crazier. What you have to keep in mind though is that you pick your primary profession with several things in mind, your armor, your energy (especially your energy return), and your primary attribute (which is applicable to only your primary profession).
I'll just link the wiki for those two professions. Both are great professions though. Elementalist is probably the most versatile profession in the game in my opinion. And Necros are a staple for most team builds.
Elementalist
Necromancer
End game is different than any other mmos pretty much. 90% of the game is played at max level. Leveling is your tutorial pretty much. At max level, you continue through your missions, gain titles, get different armor sets, work on hardmode areas, and also run through the elite areas and dungeons.