The thing stops your brains falling out when you're shot in the head.
First introduced in the British Army in the autumn of 1915, widespread issue did not occur until the spring of the following year - just in time for the wholesale slaughter on the Somme. The first type helmet was referred to as the Brodie (who designed it). The design was based on medieval headgear called the Kettle helm. Use of this type lid lasted until the end of WWII, though by 1944 the Mk.III, or '44 pattern helmet came in to use - though mainly by Canadians and assault troops in time for the Normandy landings.
Other specialist helmets were introduced for wartime use, such as parachutist and dispatch rider's variants. After the war the Mk.IV Helmet became general issue. This was a slightly redesigned Mk.III but the general appearance remained the same. This helmet remained in use until the introduction of the Mk.6 Combat Helmet in the late 1980s.
There was also the short-lived NICH Helmet (Northern Ireland Combat Helmet) that was manufactured from ballistic nylon with an airborne-style harness which was also utilised on the PARA 'jump' Helmet of the 1980s.
The British Army now uses three types: The Mk.6 Combat Helmet, the PARA Helmet and the new Mk.6a, which offers improved safety over the other two and is currently (early 2006) only being issued on operations. The PARA version is lighter than the others, much more comfortable and looks more "ally".
Health and Safety Nazis often tell people that the Kevlar Para Helmet does not offer as much ballistic protection as the Mk.6. This is because they are jealous, tending to be the sort of people who wear their Mk.6 with a FFD as padding, so it sits on top of their head like a teat on the engorged bell-end of a condom clad penis. Like as not, they are the same, pitiful, SASC choppers who appear in the small arms pamphlets with their helmet covers on sideways.
The subject of much discussion, on this thread: http://www.arrse.co.uk/cpgn2/Forums/viewtopic/t=34407.html
The origin of this debate lies in the fact that parachute troops were, at one stage, issued a fibreglass "jump" helmet with no ballistic protection. The current issue para helmet is made by NP Aerospace, the same people who make the Mk.6, and now has comparable ballistic protection. Pamphlet 21 specifically makes no distinction between para and Mk6 helmets when stipulating body armour required for LFTT.