Scales used in Western music will typically involve notes separated by whole-step or half-step intervals. Theses scales are typically either major or minor; major scales generally sound happy and joyful, while minor scales sound sad or longing. Scales are named after the first note of the grouping. For example, the C major scale is C, D, E, F, G, A and B, and the A minor scale is A, B, C, D, E, F and G.
Both of the above examples represent diatonic scales, or scales with seven notes. Another type of scale is the chromatic scale, which has 12 notes. This scale starts with any key on the piano keyboard, and ascends or descends including every black and white key until the original note is reached. Other examples of scales include the whole-tone scale, with six notes, and the pentatonic scale. A simple way of playing the pentatonic scale is by only hitting the black keys on a keyboard.
In Indian classical music, scales are called "ragas," and notes are called "swaras." Whereas in Western music a sight-singer would use solfege, such as do, re, mi, fa, so, la, ti, do, to represent a major scale, an Indian raga might include the syllables sa, ri, ga, ma, pa, dha and ni. There exists an enormous variety of ragas, defined separately by scale, intonation, ornamentation, and line of descent or ascent, as opposed to Western scale definitions which only include note values.
Although many modern composers still restrict themselves to the use of the Western scales mentioned above, many adventurous composers have devised their own unique scales. For example, Harry Partch is known for creating a 43-note scale, and even made his own instruments to be compatible with this enormous scale. John Cage wrote experimental compositions for prepared piano, in which scales could be modified and bent to his will.
When composing classical music today, it's important to remember that there are no hard and fast rules to follow. It's entirely acceptable to create your own scale or to ignore the concept of scales altogether. Modern electronic music helps to facilitate this; notes can be modified and detuned to your liking, and interesting sonic landscapes can be easily built by composers who are willing to experiment.