Profession trainers (not to be confused with skill trainers) are NPCs that will introduce you to your primary and secondary profession. They are the ones who will teach you your first skills and give you your first quests.
For an overview of the available profession trainers and the services they offer, see:
- Profession trainer (Prophecies) for the Prophecies campaign
- Profession trainer (Factions) for the Factions campaign
- Profession trainer (Nightfall) for the Nightfall campaign
- Profession Changers in each campaign can help you change your secondary profession.