Basically if you pick a talent does it restrict you to those skills?
Because you said you're a mage I'll use mage as an example.
Talents are purely beneficial, picking a talent tree will not RESTRICT you from anything. Every talent tree has 1 ability and 1 or more passive abilities associated with it from the start. For example frost comes with the spell Water Elemental and Frost Specialty. The only way to get Water Elemental and the other 2 talent abilities is through talents so you can only have one.
Once you have chosen a talent tree you can invest talent points into that tree. Depending on what you put these talents in you will get bonuses (which are almost always associated with that talent tree's abilities). For example there's a frost talent (Fingers of Frost) which has a chance to trigger on any spell that applies a chilling effect. Once it triggers, you get to cast Ice Lance instantly.