Referred to by Abhakhazia as “Britain's own little Iraq”, it's the part of the island Ireland, consisting of most (but not all) of the historic Irish province of Ulster, that remained with the United Kingdom after the independence of the rest in the 1920s. The south, initially a British dominion in name if not practice (the Irish Free State) became the Republic of Ireland in 1949; British people sometimes refer to it as Eire to differentiate the political entity from the geographic Ireland.
Northern Ireland's borders were essentially drawn to produce the largest chunk of Ireland possible that would still have a slight Protestant (i.e., Loyalist) majority. However, this incorporated a significant number of (mostly Catholic) republicans who would rather that Northern Ireland leave the Union and be (re)joined to the Republic. This is the root of the Irish 'troubles', which are generally considered to have lasted from the 1960s to (hopefully) the 1990s. The Catholic/Protestant divide tends to be quite clear out in the country, but less so in the cities, which have sometimes ghettoised into rival neighbourhoods with propaganda graffiti attacking each other.
Northern Ireland is notable in that it's almost impossible to mention any of the names of the places in it without upsetting someone, e.g. Londonderry vs Derry. Even the unofficial national anthem is variously known as either 'Danny Boy' or 'Londonderry Air'. Although the region is obviously part of the UK, using the word 'British' to describe it is also politically charged.
Aside from politics, Northern Ireland is home to the largest lake in the British Isles, Lough Neagh.