Octavius is right. however, I'll clarify. If you buy a leasehold property you are buying the building (bricks, mortor, etc) you are then responsible for this, you usually pay a maintenance charge for communial areas gardens, hallways, lifts, etc. If you buy freehold you purchase the property and land outright and are responsible for all of it.
If you buy leasehold the owner of the leasehold can increase the lease (fee) for the land or maintence this is usually for a term of 99 years.
If you buy leasehold you can always buy the leasehold off the owner making the property freehold.
If you are considering buying a leasehold property you should check the terms of the lease and at what date the current lease expires. If you are buying an a flat / apartment you would need to ensure that all residents comply and that they are bound to contributre to any expenditure.
In short (to late), buy houses freehold and flats / apartments leasehold, where possible.