Depending on the size and the market of the firm, M&A can be its own practice, or part of a larger general securities practice. Many firms now subdivide their corporate practice by industry (healthcare, technology, oil & gas, etc.). My firm has a general corporate finance department that does a bit of everything, but each partner tends to informally run a certain practice area on his own. For example, the UK partner I work with does predominantly CIS deals, but he will do every type of corporate deal imaginable - IPO's, tenders, M&A, private placements, etc.
You can learn a lot from a firm's website, but it's also a good idea to ask these questions when you are on an interview.