Basically a veve is a symbol that represents a specific loa or spirit and is used within Vodoun religious practices.
The loa is invited to take possession of a human body, only temporarily so that they can then communicate directly with them. This is usually done with chants, drum beats, colours, offerings and the drawing of veves to invite the loa in, all of which are specific to the loa that is desired.
The veve is usually drawn on the ground using a powder of some kind, it might be cornmeal or sand perhaps. It serves as a focal point for offerings and invocation.
I have some of the veve symbols printed off from the internet that I use in workings, they are also good inscribed on candles to add power.
Some of them are just beautiful to look at.