An investment trust is a company whose sole business is to manage the investments it owns. The management of an investment trust may be employees of the trust (in which case it is called a self-managed trust) or, more commonly, may be an external fund management company.
The use of the term trust is slightly misleading (and largely confined to Britain and the Commonwealth) as there is no trustee involved. It reflects the historical origins of investment trusts.
The shares of investment trusts are listed and therefore they are bought and sold through a broker, like the shares of any listed company. There is usually an annual management charge that is paid out of the investment trust's assets.
Unlike OEICs, investment trusts are closed ended so the amount of money in the fund does not change; investors cannot withdraw money from an investment trust, or add to the money in the trust: they buy or sell its shares.
This means that investment trusts usually trade a discount to the value of their assets. A particularly well regarded trust may trade at a premium, one about which the market is very pessimistic is likely to trade at a large discount. The discounts can lead to an extra gain if the trust is wound up.
More information (obviously not from a particularly disinterested perspective) can be obtained from the Association of Investment Trust Companies.