A unique attack ("特殊技", Tokushuwaza, "Special Technique" or "Unique Art") as implied by its name, is an attack "unique" to the character using it. Unique attacks were introduced in the Street Fighter II series. In later games and other series, such as the King of Fighters, they are informally known as "command normals".
A unique attack is usually performed with a simple motion (e.g. forward, down-forward, back, etc.) and a button, usually a specific one (e.g. medium punch, light kick). Most characters have at least one unique attack. Though some characters may have the same input for their unique attacks, the actual attacks themseles differ with the fighter. Despite this (and the name), it is possible to see two fighters with the same unique attack (e.g. Spin Back Knuckle); this may be due in part to them using similar enough fighting styles (e.g. Guile and Charlie, in relation to the previous example).
They tend to be used often for several combo strings, due to how most of them can be reliably chained into and canceled out of. Most of them can be chained into special moves, but some of them can only be chained a certain category of attacks (such as a heavy attack), some cannot be chained into at all, some are not cancelable into special moves, and so forth.
For a list of unique attacks, see Category:Unique Attacks.