A clerk is catchall description for someone who carries out general administrative work in an office. The job itself can be quite broad in terms of what's expected, though it usually involves some form of bookkeeping and admin work. It is generally the first position someone will take at a company and is often filled with ambitious employees willing to work their way up the corporate ladder. In America the word clerk can also mean a shop assistant but in Britain it doesn't carry that connotation.
A clerk's responsibilities can be pretty varied. The role is often defined by the sector someone is working in but usually involves bookkeeping, filing, the occasional errand and other administrative tasks. A clerk is often the lowest rung on the corporate ladder and so is often be the first to come under scrutiny if things go wrong. They must show the company that they cannot do without them and so must be reliable, dedicated and extremely hard working, often sacrificing their own personal life to offer to work longer hours than required. Accuracy and analytical skills are also paramount to anyone wanting to work in such a position.