In general, the term "practice area" is used to distinguish legal teams based on the nature of the legal services they provide (e.g. corporate advice, intellectual property advice, advice on disputes, and so on). 

Each team within a firm tends to focus on different types of legal documents and processes, and they are classified into different practice areas to make the distinction clear. 

“Sectors” on the other hand refers to the industries – or “categories” of clients - that lawyers deliver services to, for example clients that operate in the energy, music, transport, healthcare or fashion industries. 

It can be a little confusing at times. For example, "real estate" is a common law firm practice area, but might also refer to the property development industry. Finance or banking is another common law firm practice area, but "financial services" is also a term used to describe the broader banking industry. In these types of cases, the phrases might be used to refer to both practice areas and sectors.

Helpfully, many firms distinguish between practice areas (which are often called “services”) and sectors (which are often called “industries”) on their websites, so this should give you a helpful steer. 

Sometimes firms do use terms like "capabilities", which can be a bit confusing. However, when pitching to clients (which is the main point of most firms' websites), it's often more effective for firms to say things like "we have a world-leading energy team”, or "we're real technology and Blockchain specialists", than it is to say "we do corporate, finance, IP and disputes work", as the former sounds more tailored to clients (who might not really understand or care about how their legal needs fall across various internal practice areas).

Lawyers might also sometimes say things like "I'm a tech lawyer". But what they often really mean is that they carry out legal work linked to their specific practice area (for example, corporate or real estate), but  frequently for technology clients.

To summarise, the term “practice area” relates to what lawyers actually do on a day-to-day basis, whereas the term “sector” relates to the clients to which lawyers deliver services.