You can find all about the Character Class Tree here

When you begin the game - you are entered into the Cave in Bootcamp - originally you were on the otherside of the map, until the Bane overrann the Control point, destroying practically everything, killing everyone they could. But still the AFS stayed, and chose the Cave with the hologram Eloh...
You start off at Level 1 and work up to Level 4 as a Recruit.
Here you reach your first branch, do you choose the Soldier path, or the Specialist path?
You stay as either class from Level 5 till Level 14. If you chose the Soldier Path, you'd then branch off to either a Commando or a Ranger. However if you chose the Specialist path, you'd branch off to, Sapper or Biotechnician. From here you reach for final part of the tree, and one of the hardest for your main character...
You followed one of the branches and at Level 30 you can branch to either of the final part where you stay that class forever up to the max Level of 50.
Following Commando to a Grenadier or Guardian
Following Ranger to a Sniper or Spy
Following Sapper to a Demolitionist or Engineer
Following Biotechnician to Medic or Exobiologist


Remember once you reach each branch you can clone a character - so you can be a Solider and specialist at the same time!