In Brood War, as only a single worker can mine from a patch of minerals, sending your 4 initial workers to one patch leaves the other 3 idling until the first is done. Therefore, "splitting" your initial workers to mine from different patches greatly boosts your early-game income.
In SC2, the AI splits workers automatically and nearly optimally, yet some players continue to do it manually.
Upon sending a worker to a patch that is being mined from, StarCraft II's AI will automatically assign it to go mine from a patch that is not being mined from. Therefore, sending your initial 6 workers to a single patch will trigger the AI to automatically send the 5 remaining workers to other patches.
This is by far the easiest way to split, but a disadvantage is that you cannot assign workers to mine from the optimal patches. Also it is not the fastest way to start mining as workers will first touch upon the patch assigned by yourself before moving on to the next.
Pressingwill select one of your idle workers. By spamming and right-click you can send all your workers to a separate mineral patch.
This way of splitting will allow you to select the optimal patches to be mined from and, as a result of splitting all your workers, mining will be a tad faster than the AI Split.
The most popular splitting method is splitting off. You send the initial 6 workers to get them moving. While they're moving you select some of the workers and send them to different patches - much like how it was done in StarCraft: Brood War. It is hard to do as the moving workers clump together - making it difficult to select individual ones. Popular alternatives are splitting 3 by 3, possibly splitting up on threesome further.
For an in-depth comparison between these three split methods, have a look at this thread from the TeamLiquid Forums: Worker splitting: Does it really matter?