The question 'What is land?' might seem overly simplistic but in a legal context comprises a range of factors. The best way to think of land is in 5 dimensions:
1) xaxis
2) yaxis
3) zaxis
4) Time
5) Equity

The definition of land comes from s. 205(1)(ix) of the Law of Property Act 1925 but apart from adding information about corporeal and incorporeal hereditaments it is not of great use.

1 and 2) The x and yaxis is how land appears on a map. However maps such as the ordnance survey are not definitive as to boundaries. This can cause boundary disputes that cost a lot of money and take a lot of court time. There are common law rules in place such as ad medium felium that can help resolve such disputes. Alluvion and deluvion can also affect the boundaries of a piece of land.

3) On the zaxis it was originally stated that 'Cuius est solum, eius est usque ad coelum et ad inferos' which means that land would extend from heaven to hell. Nowadays people have limited rights to things like oil, gas, coal, gold and silver beneath their land although they do have rights to minerals. Subterranean trespass is also something to consider.
Above ground there are limitations in terms of a range of factors such as aircraft, sunlight, leylandii, treasure (Treasure Act 1996), trees, plants, flowers, wild animals, birds and water.
Fixtures are another important consideration though it can be hard to contrast with chattels. One way to make the contrast is by looking at the judgment of Blackburn J in Holland v Hodgson (1872) that builds on the phrase 'Quicquid plantatur solo, solo cedit' by looking at the mode, degree and purpose of the annexation.

4) People in England and Wales cannot technically own land as all land is owned by the Crown. However people can own estates in land and the fee simple absolute effectively gives ownership for an unlimited period of time. On the other hand leasehold estates give ownership rights for a limited (but often long) period of time.

5) The final dimension take account of equity and trusts and examines legal and beneficial ownership of land. Ultimately equity ensures people deal with land in a conscionable fashion.