Did not get around the theory!

Although it is easy to compose music with Sibelius, we still did have to learn one or two theoretical things.

We discussed the four main elements for music composition:

Rhythm

is the beat or tempo of the music which is determined by the time signature and note lengths (beats).

Source: http://sureshsingaratnam.com/blog/?p=530

The time signature defines how many beats are within a bar.
A waltz, for example, has a time signature of 3/4, that means that the beat is counted "one, two, three, one, two, three" and so on, with the the accent being on one. A 4/4 beat is likewise counted "one, two, three, four, one, two, three, four" with the accent on one and less strongly on three.

So a 3/4 time means that there are 3 quarter notes within the bar, but there could be any other note lengths as long as they add up to 3 quarter notes.

Source: http://www.free-easy-piano-lessons.com/time-signatures.html

There are some good explanations on the website below:
http://www.studybass.com/lessons/reading-music/time-signatures/

Melody

My explanation would have been that melody is a succession of tones or sounds that have a different pitch, but the scientific explanation is much more prosaic:
Pitch is an auditory sensation in which a listener assigns musical tones to relative positions on a musical scale based primarily on the frequency of vibration. For more on oscillation, hertz and cycles, go to the Wikepedia article, can't really get my head around it.

Source: Wikipedia

Harmony

is the consonance or dissonance of two or more notes played together at the same time. Our European or western perception of what sounds harmonious is different from what other cultures might consider harmonious. Western music is based on a chromatic scale consisting of 12 tones (C, C#, D, D#, E, F, F#, G, G#, A, A# and B, which is an octave).
A key signature (scale) is made up of 8 notes, for example C-major starts with the middle C, then D,E,F,G,A,B,C. All the keys, no matter on which tone they are based, always have a half step between the 3rd and 4th and 7th and 8th notes and full steps between all the others.
Good explanations:
http://www.didjshop.com/BasicMusicalHarmony.html

Timbre

is whether the music feels happy or sombre and sad. In western cultures, minor intervals are usually associated with sadness, thoughtfulness or interiorness while major ones have been related to feelings of joy, happiness, brilliance, etc