It is the process of testing different versions of something, so that you can improve your conversions. A/B means having two versions, that are tested against each other.
Could be:
Squeeze pages
Sales Pages
Optin Boxes
Advertisements/banners
Blog post titles
Email subject lines
ect, ect... basically any copy
You have version A and version B, the traffic is split between the two. After enough traffic you can see which performed better. use the best as a control and set up another version to test again.
For proper testing, the versions should be pretty much the same, except for one difference, such as a different headline. If you changed more than one part, you won't know which change made the difference, good or bad.
There is one other idea, I heard some time back, though I have never tried it. That is, instead of two versions with only one change, having two versions that are radically different. It could speed up the process. Sounds like a good idea, just never tried it.
Split testing could be and should be an ongoing test, most everything can always be improved.
It's something every marketer should be doing, though many don't. I know I don't test everything I should or could.
Hope that makes sense and helps.
