The majority of training contracts available each year are at law firms, so it's no wonder that most candidates concentrate their applications on these. There are in-house training contracts available though - here's what you need to know about them.
What will I be doing? The Solicitors Regulation Authority requires all trainees (whether in-house or in private practice) to undertake work in at least three different areas of English law. In private practice, this typically entails trainees physically moving offices ("seats") and changing supervisors every six months to experience different departments. In-house, where there is likely only one legal department, you (or your allocated supervisor) will need to seek out different kinds of work to satisfy this requirement.
Being part of the legal team of a commercial organisation or local authority (the largest in-house employers) means that you'll be advising the business day to day on legal aspects of the work they do, drafting and reviewing contracts, preparing/updating company policy in line with legislation, and possibly also training others on legal requirements they need to adhere to.
Any advantages? You'll be far more business-focused and commercially aware than your private practice colleagues, since your daily interactions will be with the commercial staff you advise.
You may also find yourself instructing law firms (see below), for whom you are in effect, the client - expect to be flattered and sucked up to in their pursuit of more work from your team.
Any disadvantages? The stereotype of in-house work is that all the interesting work gets farmed out to specialist law firms. It's not entirely true, but you will typically find that some of the more complex (and therefore juicier) stuff passes you by.
Pay is typically lower than private practice, and tends to increase more slowly, though you may be eligible for bonuses and perks.
What happens when I qualify? Typically you'd qualify into the legal team where you trained, though (like all NQs) you'd be free to look around for a better deal. You might find it harder to demonstrate the transferability of your skills if you've worked in a niche area.
Is it worth applying? Up to you. If you want a business-focused TC in a small team with early responsibility and consistent hours, training in-house might be for you.
Here are some resources to help you learn more:
In-house training contracts (All about law)
Alternative careers in the law (Chambers student)
Your guide to becoming an in-house lawyer (Guardian)