The purpose of 3-d glasses is to completely separate the image for the right eye from the image for the left eye. This separation is necessary for stereo (3-d) sight.
The most common type of 3-d glasses are anaglyph (red/blue) glasses. With this type of glasses, a red filter on the left and a blue filter is on the right. This is the type of 3-d glasses that were used with 3-d movies in the 1950's and are still used with comic books and magazines today. For example, this year's Sports Illustrated Swimsuit Edition has a couple of 3-d layouts (for purely artistic reasons, I assure you.)
Another type of 3-d glasses uses polarized lenses. The two lenses are polarized perpendicular to one another. The old Captain EO movie at Disneyland used this sort of glasses.
The final type of 3-d glasses is used for stereo photography. In stereo photography, prints are developed as a pair of side-by-side images. This type of 3-d glasses (often called "Holmes viewers" after their inventor, Oliver Wendell Holmes) use prisms to keep the eyes' line of sight parallel to one another so they can focus on both images simultaneously.