The term toccata is Latin for "touch." This is an apt name, as they are written to show off the technical skill of a pianist. Unlike other, more virtuoso-esque, forms of music, toccatas are not composed for sheer power, but agility and control. In fact, toccatas are among the most dynamic types of music to be written; typified by drastically changing rhythms, harmonies, melodies, and moods.
First appearing in the late 15th century, toccatas have been seen throughout history since then. Some of the first major composers to embrace them were the Venetians Giovanni Gabrieli and Claudio Merulo. They wrote toccatas for the organ, setting the stage for later (Baroque) developments of the form. Merulo also began alternating toccata and fugal passages. As fugal works are more level (thematic and unchanging), this was a mixing of virtually opposite forms of music. Later in history, however (especially with J.S. Bach), this practice would become common.
In the mid 17th century, Girolamo Frescobaldi, an Italian, included improvisation into the toccata. Still constantly changing throughout, each of these pieces were meant to be played loosely, in their entirety, or separately in sections. Johann Jakob Froberger, who studied with Frescobaldi, helped bring the toccata to Germany and continued the integration of fugal passages into the form.
The toccata greatly intrigued those of the Baroque period. The variety, and use of contrasting ideas, appealed to Baroque values (i.e. unity in opposites) and became very popular. This was evoked in J.S. Bach's works where a toccata would often take the place of a prelude before a fugue. He made heavy use of the form, and fortified its position throughout all following periods of music.
Some later composers of toccatas include Claude Debussy (French impressionism - Pour le Piano at the turn of the 20th century) and Francis Poulenc (French 20th century - Trois Pieces).