Can one character play any role in this game?

Hi I'm new to the game. Usually in mmorpgs, I usually have 3 characters. One dps, one healer and one tank so I can enjoy the game in every role. From what I read on the job/class system, you can swap to anything you want? So if I'm a conjurer right now, could I spec into a tank and do dungeons without issues?

Yes, you gain each class by joining the guild for the class. Once you gain a class, switching is as easy as changing you're main hand item. You may need to do some re-equipping of gear when you switch, but once you have everything equipped you can create a gear set for the class and easily switch back and forth on the fly.

Functionally, yes. Class levels are retained when you change, so nothing stops you from having each class leveled and somewhat geared.

Practically, depends. The highest tiers of gear (i100 and i110 currently) are time-gated in that you only get so many (chances) at them per week among all of your jobs. This usually forces people to main one job for endgame raiding, until they or their group (if they have a static group) have enough weekly currency to start funding a 2nd job or enough repeat/unwanted drops are accumulated that nobody minds a secondary getting a piece.

If you don't care about the absolute highest tier of gear (and until you reach that point), gear up to i90 is pretty freely accessible.

When you finish a lvl 10 class quest, you unlock ability to join other classes as well as gear sets. As long as you save a gear set saved for every class you want to play changing a class is as simple as clicking one button. The more classes you unlock, the more gear sets you will be able to save. You can also put gear sets on a hotbar.

Once you get to level 10 on your first class, you can swap to any other class you want as long as you've been to that class's guild to unlock it. I believe you can do the airship quest at 15 or so, and that'll give you access to guilds outside of your starting city. Make sure to research and level alternate classes up too so that you can open jobs once you hit 30.