A scale is simply a progression of notes in ascending or descending order, so arranged for theoretical purposes, vocal or instrumental exercise, or as part of a composition. A scale consists of a certain number of divisions of a fourth (Tetrachord) or Octave.
Before we continue, an Octave is the interval between the first and eighth notes of a diatonic scale. A diatonic scale will be defined later, but in simpler terms, notes that are an octave apart are called by the same letter name. A note at the end of one octave will have exactly twice the number of frequency vibrations of the lower note. An example of one octave in the C Major scale is shown below.
The manner of the divisions within a scale is called the MODE. The word MODE and how its derived is difficult to understand and out of this web page scope, but, in may cases the word MODE simply refers to Pentatonic Scales (major and minor scales), and Chromatic and Whole Tone scale(s).
A pentatonic scale has five notes in an octave, eg. C-D-F-G-A. This scale type is used in the 
traditional music of China, Japan, the Far East and Africa. It is also used in some Scottish and Irish folk songs, in
the US Negro Spirituals and by some 20th centry composers, such as Debussy and Ravel, to achieve an
oriental effect in their music.
A chromatic scale is a scale that is proceeded entirely by semitones. A semitone is the smallest interval 
(raised or lowered) between whole 'tones', thus, it is also called a 'half a tone'. There is a total
of twelve semitones in an octave. The semitone is the smallest interval used in western style music.
As the name suggests, a whole-tone scale consists of a series of intervals of one whole tone (or note). A
whole note is the US term for a semibreve. There are no tonic (first note of scale, eg. C in the C major scale)
or any other implicit relationship between the notes of a series within a whole-tone scale. Two such scales are possible, the notation of which may vary according to convenience: C-D-E-F-G#-A#-B#(or C), and,
D flat-E flat-F-G-A-B-C# (or D flat). The scale starting on D is obviously part of the same series
as the one starting on C, and so on. Debussy and other composers have featured whole-tone scales extensively
in their music, both harmonically and melodically.
The word diatonic in its strict sence, is the use of notes that are proper to key. For example, in the key of D major (with F#, C#) the following notes are diatonic and constitute diatonic scale: D-E-F#-G-A-B-C#-D. Any other notes involve accidentals and are chromatic.
In minor keys, the sharpened sixth and seventh notes are in such common use, and though not strictly proper to key, that they are also regarded as diatonic. For example, the key of D minor, notes B natural and C# are added to the proper diatonic scale, thus forming: D-E-F-G-A-B flat-C-D.
The modern diatonic scale has two distinct modes (note patterns), they being :
(note, T = tone, S = semitone)
The minor mode has more of a theoretical existance, and in practice consists of two forms:
The similarities (and differences) between major, minor (harmonic and melodic) scales can be quite confusing and I can always remember at the age of 14, having a hell of a time trying to fathom it all. So, here are a few examples and some general rules to follow when determining the key and accidentials within a scale.
David Webber 1994-96. Mozart Music Generator Program
Concise Dictionary of Music, 1986.
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