It does taste like celery, although in my opinion, a lot milder. You can eat it both raw &/or cooked.
To enjoy it raw in salads or, as Ishbel mentioned, in the classic "Celeriac Remoulade" (which is really just raw celeriac tossed with remoulade sauce - or russian salad dressing in a pinch - lol!) you really do need to peel & slice it into julienne matchsticks, because it can be a bit fibrous.
For cooked dishes, while you still need to peel it, you can use it the same way in which you'd use other root vegetables - gratins, mashed, purees, etc.