Canards can be added to planes whose center of lift is too far behind the center of mass and thus “excessively stable” in whichever heading they feel like going. They are also useful as pitch, thus angle-of-attack, increases because canards will tend to stall sooner than the main wing, resulting in less lift on the nose and a more level flight orientation, probably.
Since version 1.0, all parts generate lift, so a canard can in theory be any part. However, smaller wings are best used for static canards. There can be complex interactions between static canards and likewise static main wings can lead to issues with stability in aerodynamic stalls. Active canards made with control surfaces, and in particular controlled wings like the AV-R8 Winglet and the Advanced and the Standard Canard, can actually increase maneuverability and stall performance.
Canard on Wikipedia