Here are a couple of things to think about:
4 main periods -
Baroque (steady tempo, no pedal, elaborate polyphonic melodies). Charcterised by Bach, Handel
Classical (elegant melodies, regular harmoies). Characterised by Mozart, Haydn.
Romantic (lots of dynamics, pedal, big slushy tunes, rubato). Liszt, Chopin, Rachmanninov
Modern (choppy melody, irregular harmonies, syncopation) Bartok, Stravinsky
(a small but very distinctive modern style was ragtime - syncopated but simple tunes - eg Joplin)
Dynamics - were there any? Did it start quiet, end loud, fade away? etc.
Rhythm - use of dotted notes, syncopation. Simple or compound time?
General - Does it sound like a song? A dance? (extra brownie points if you know the name of the dance - minuet, gavotte etc). Was the melody in the left hand? Was it fugal? Did any part repeat? Length of phrases?
If they ask for a possible composer at the end don't get too hung up of you don't know. What they are basically after is that you recognise the period the music is from and can name a composer of the right era. Just make sure you have a few likely names to hand.
Remember that the musical discussion is only one part of the aural, which in itself is only 18 out of the total 150 marks. They will only ask about 3 questions, and won't be expecting a long detailed discussion, since the whole exam will be over in less than half an hour.
Hope this helps!