If my ap (say) picks up an exotic and wields it in his primary hand, he will see a skill that looks something like rip 1% (50%).

If he improves it by using it, it'll become rip 2% (51%).

When he unwields it he will still have rip at 2%. He can only use the skill though if wielding, in his primary, any exotic weapon that grants the rip skill. The amount of extra skill added by the weapon can vary by weapon. One might give you rip 2% (10%), while another gives you the 2% (51%).

In my experience some skills like rip are easy to improve and do more as you get better at them. At low skill you get just damage. Then you get damage and chance of causing bleeding. Then you get damage, chance of bleeding, and chance of disarm.

Others seem really hard to improve, like projection, lunge, and for me who tends to play low strength races, wallop and batter.