Bandwidth may be Burstable or capped at client's discretion

Whats the difference, which one is better?

If you have a 256kbit cap, you can never transmit faster than 256kbit/s (roughly 25-28KBytes/second).

If you have a 256kbit "limit" but it is burstable, you can burst up to your line's full speed (if you're on a T3, that can be VERY fast) and provide more speed to more users at one time. With a capped line you can't.

Hope that made sense. :)

And when its burstable and you cross that limit will you loose more bandwidht..or it has nothing to do with that ( it ll just be faster than normal) Will that cause more $$ ?

As long as your monthly average or 95th percentile measurement (whichever your provider measures by) is less than your limit, you're fine. You can burst to 1.5mbit on a 256kbit limit for a short amount of time and still be fine. You should double check with your provider, of course, in case they deviate from the norm for some reason.