What's a "K-Factor"?

That's the name of Wilson's rackets...quite a new technology...

the letter K comes from the material that is used to make these rackets...80% Karophite Black and 20% Kevlar...

the numbers like 90, 105 etc indicate the overall size of the racket in inches...players like Federer use smaller rackets ..i.e...the 90 sq inch ones while players like Serena use the 105 sq inch ones...all these K factor rackets are used generally by players with a rating of 5.0+