The simplest answer is: it is the wall separating two buildings. Each neighbour owns half of the wall but has rights over the other half
If one neighbour wants to do some works that affect the party wall, such as remove a chimney breast or install a beam, they need to let the other neighbour know (via a Party Wall Notice) and carry out the works in such away to minimise the effects to the wall and the neighbour. This is all agreed up-front and in writing and recorded in document called a Party Wall Award. Once finalised the Party Wall Award is a legal document and should be kept safe with the house deeds
Are There Any Other Types of Party Wall?
There are no other types of Party Wall but the Party Wall Act does cover i. building new walls on or near a boundary and ii. digging foundations etc near other buildings
i. if you want to build a new wall on or astride the boundary of your property you need to tell your neighbour and let them know the details (via a Line of Junction Notice). Then the details of the wall and how to minimise the disturbance etc to the neighbour are agreed in the Party Wall Award
ii. if you are digging foundations or drains etc within 3 metres of neighbouring building AND digging down deeper that your neighbour's foundations you need to let the neighbour know the details (via a Notice of Adjacent Excavation). Again the details of the works, how to protect the neighbour's building and how to minimise the disturbance to the neighbour are agreed up-front in writing and recorded in the Party Wall Award
iii. There is a second, rarer, type of Notice of Adjacent Excavation; where you are digging within 6 metres of a neighbour's building and the excavation is significantly deeper than the neighbours foundations. It follows the same procedure as the above but is rare in domestic situations
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