As a general rule of thumb, most houses are freehold and most flats/apartments are leasehold. However, as you will have seen from the above, there is now a trend of also having leasehold houses on estates.
The reasons for this is usually so that the owner of the whole block of flats/estate of houses, has control over the common parts of the estate or building (so the roads or stairwells by example) and can then charge a service charge to the individual leaseholders to look after those common parts. It is possible for these sort of arrangements to also apply to freeholds (by virtue of grant of rights and payment for those rights, but that is not that common).
Leases will also usually have restrictions on what you can do with the property - i.e. alterations, and in some cases decoration. Leases will also usually have some obligations on your landlord to look after certain things for you, i.e. they may for example be responsible for sorting out the boiler when it goes wrong, or repairing damage caused by leaks. If you are buying a leasehold you should check the terms of the lease carefully. You should also check the length of the lease remaining - generally speaking leases with less than 60 years to run cannot be mortgaged, so you would only be able to buy those with equity. As a pp mentioned, some leases have a right in statute to be extended/exchanged for a freehold if certain conditions are met.
As a PP said, both freeholds and leaseholds have their pros and cons; it depends on what you want.
What I will say is that the demise is not the distinguishing factor between the two - it is the length of time the right to the property is granted. A freehold is forever (well, technically until the queen wants it back - but that isn't viewed seriously) and a leasehold is for whatever length of time the lease is granted. Whilst it is more usual that a freehold will include the land as well as any structures on it, it doesn't have to. You could have a freehold of just a house and not the land it's on (though that would be very unusual) and a lease of a house and all the land. You should check or ask your lawyer to check the plans for any property you wish to buy so you can see what is included or not.