Bit of a broad question, Donna.
There are lots of different roles - even as a Personnel Administrator, which is where one would probably expect to start out. The larger the company, the more specialist the roles - for both Administrators and HR Managers.
HR Administrators may cover:
Supporting the recruitment process, including doing some interviewing of lower level jobs
Staff induction
Supporting the HR department with HR stats, including sourcing the data
Pensions administration (a specialist area)
Supporting managers with admin, including writing letters to employees on pay and discipline matters (amongst many others), communications events
Supporting training - inhouse or external including records
The CIPD is one of better sources for training support to HR - see here.
http://www.cipd.co.uk/training/