I would say theory at its most basic is an explanation of what the symbols on a page of music mean: that a half note (minim) takes twice as long to play as a quarter note (crotchet), for instance. Slightly more advanced theory would deal with things like the key of a piece -- which itself has to do with what "family" of notes the melody could contain and which chords are most likely to be used.
A scale is a (particular) succession of notes played in a certain order. The one usually first introduced to beginners (at the piano) is the C major scale: play all the white keys, starting on one C and ending on the next for a one octave scale. (C D E F G A B C ascending; C B A G F E D C descending.)
The most common types of scales are major and minor. All major scales have a similar sound, because all follow the same pattern of tones and semitones. (A semitone is the distance from one key to the very next key, for example F to F#. A tone is two semitones, for example F to G).
I can tell you about minor scales, chromatics, pentatonic scales, etc. another time if you're really interested.
Alice
Topic: BEGINER thread