The use of the term ‘contractor’ tends to describe a situation where a lawyer or solicitor is working on an ongoing basis, usually to deal with excess capacity or a busy period.
The use of the word ‘locum’ tends to be used when referring to a lawyer or solicitor covering for sick leave, maternity cover, replacement cover (during notice periods) or any short term cover.
In the IT industry quite a few skilled employees are used on a contractor basis to deal with specific projects. This does not tend to happen with the legal profession – projects per se do not tend to crop up. It is usually cover for specific incidents or periods of time that is more relevant.
Locums work on a self-employed basis. In other industries contractors work on an employed basis via the employment agency who introduced them. In law firms contractors work on a self-employed basis as well, not usually via agencies unless dealing with a large multinational.