Freehold
If you buy a house you will usually buy the freehold. This means you will be the outright owner.
Leasehold
If you buy a flat or maisonette, you will become a leaseholder. This means that you and your successors can live in the flat for a fixed time, usually 125 years. You will be responsible for the interior of your home and the Council will be responsible for looking after the structure and exterior of the block (including the windows). You will have to pay for major repair and improvement works through service charges. These can be high, especially if the block needs major work.
Leaseholders can sell their property at any point with the buyer taking over the remainder of the lease. So, if you sell after 15 years, the buyer will get a 110 year lease.