10 mbps means you are transferring 10 million bits of data every second.
100 mbps means you are transferring 100 million bits of data every second.
A 10 / 100 card means it can do both.
Your Hub and cabling must be capable of 100mbps as well before you can get the speed.
Bearing in mind a 56K modem does 56 thousand bits a second and even cable is only 500 thousand a second a 10mbps (10 million) is fast enough for most users unless you want to transfer large graphic type files.
10BASET = 10mbps Baseband over Twisted Copper Wires.