Snapping comes in three flavors: grid snapping, context snapping, and directional snapping.
In general, you turn a snapping mode on by pressing one of the buttons in the Snap field. The buttons are independent, you can turn them on and off independently. (The snapping modes, however, are not independent. See below for the precise interaction.) Note that the keyboard shortcuts are printed on the buttons. They are rather convenient since you will want to toggle snapping modes on and off while in the middle of creating or editing some object.
Whenever one of the snapping modes is enabled, you will see a little
cross near the cursor position. This is the secondary cursor Fifi
(called after the dog in the rogue
computer game installed on
most Unix systems in the 1980's, because it also keeps running around
your feet). Fifi marks the position the mouse is snapped to. Some
people don't like Fifi (or maybe they don't like tripping on its
tail). Fifi can be turned off from the configuration
panel.