1:1 displays the image as the exact same size in real life, meanwhile 2:1 would be twice as big, 5:1 5x, etc..
The 100 2.8 Macro (L and non L variants) along with the Sigma/Tamron/Tokina 100mms and the MPE-65 are the only lenses able to go 1:1, with the MP-E being able to go up to 5:1. You can increase the magnification by adding extension tubes, which are pretty much a bunch of tubes without anything between them placing your lens further away from the camera sensor, forcing it to magnify your subject more. There are pretty much 2 types of tubes..
a) Electrical contacts- E.g. Kenko, Pro-Optic. Cost a lot more but you can change your aperture when they're mounted
b) Non-electrical contacts- E.g. cheapo Ebay Tubes. VERY cheap, often about only $3, but you have to remount your lens and hold down the DOF preview button, remove the lens and remount with tubes, everytime you want to change the aperture.
A full set of tubes on a 1:1 lens will get you about 1.68:1 (someone confirm for me please?), but then I remember someone telling me about how it's actually about 1.9-2 : 1 due to some min focusing distance thing..
Or you could always take a regular lens, get a reverse adapter and mount it backwards and add some tubes. Only drawback is you have to remove your lens and remount properly everytime you change your aperture (annoying), but I heard that the 50 1.8 reversed + tubes can do up to 3:1.. (again, unsure, but look in the Macro section, there is a thread dedicated to reversed 50mm shots)