A key is the scale in which a work is written, and the name of the key or scale is the same as the first note (the tonic) of the scale. A scale can begin on any of the twelve tones of the chromatic scale, and thus there are twelve major and twelve minor keys. The key of a piece is indicated by the key signature, which is a group of sharp or flat signs at the beginning of each staff.
For example, Beethoven's Minuet in G is written in the key of G. If the word minor is not included, such as in this case, the scale is presumed to be major. Another example is Tchaikovsky's Piano Concerto No. 1 in B flat minor.
A second meaning of the word key in a music context is the parts of a keyboard, such as that on a piano, that are pressed with the fingers to produce sound.