Unlike a freehold owner, a leasehold owner does not own the land the property is built upon. Instead they own the property under the terms of a document called a Lease for a set number of years. They also usually pay a rent to the separate freehold owner known as the landlord. The landlord will usually charge the leaseholder a service charge for maintaining the main structure of the building and common areas. This is the most common set up in a block of flats of two or more. Your lease will contain a list of rights and obligations which dictate what you can and cannot do.
Occasionally the freehold can be owned by the leaseholder too. This is usually where the flat owners have got together and purchased the freehold and each have an equal share in it. This can make life easier as you will be in control of the costs of maintaining and insuring the building. In this situation the leases are still left in place so you will have a leasehold and freehold ownership. This is to keep all the separate obligations by flat owners and landlords alike in place. Freehold flats without the leases are not usually acceptable for lending purposes.