Optical illusions are visual effects which make the viewer see something different than first anticipated, such as Magic Eye pictures. These effects usually rely on the way that the eye and the brain relay information to each other our brains often take shortcuts in order to replicate images as quickly as possible but this means that our senses aren't as reliable as we might expect.
Our brain is constantly taking in information from the world around us and comparing it to what we already know or are familiar with. To keep up, it ignores extraneous data and assumes things without taking the time to double check. Most of the time this works, but it means it can be fooled into seeing things as we either want them to be or how we think they should be.
A classic example is the inverted face mask. Because we "know" faces cannot be inverted, the brain assumes it isn't and we see a face as normal. As it spins, we get freaked out every time it comes around and we realise we have been fooled. Magicians and illusionists rely on these brain functions all the time.