So the Profession establishes some core concepts and abilities that will follow you throughout your 20 levels, along with the basics of your Wound progression, the Defensive Tests that from level 1 you get to add your Action Bonus too when you have to roll them (every profession has two of these, and some focuses provide a third), and the skills you will be allowed to pick form for skill advancement, along with telling you how many you get to assign at base level 1 character creation.

You also pick your Focus right away at 1st level. Focuses are basically the space within the wider umbrella of the profession that you specialize in. These grant you more unique and specialized abilities based on their theme, they also may add a skill or two to your options. Some even give limited magick access to professions that wouldn't normally get it.