what are the roles in the army

For more information on roles go to the British Army role finder page - you can google it.
The army is made up of 33% infantry (the role people seem to assume the whole of the army is) and the rest of the jobs are basically anything you can think of (except midwifery and hairdresser ;) )and probably more like musician and farrier. The Army is self sufficient so the jobs you have in civi street are the jobs you'd have in the army - and get qualified to do them whilst getting paid a good salary.
I notice that you've asked Reserves. If that's what you're interested in then the roles are the same but for some you'd expected to already be skilled in the job you were interested at and it may also depend what Army Reserve unit was nearest to your home or work.
The Regular army is the full time soldiers (the Army is their full time job) and Reserves is for those who might just want to dip in and dip out of the army but may want to make a difference, serve their country or just do something a bit different in their life whilst working or if they have family commitments that they don't feel are conducive to joining the army full time. I hope this helps to answer your question.