The Long, Long Trail - The British Army in the Great War was an invaluable tool as I tried to do some background research on my grandfather's WWI POW experiences http://www.1914-1918.net/
1914 - call issued for volunteers to join the army
May 1915 - upper age limit raised to 40
15 July 1915 - National Registration Act - men between the ages of 15 and 65 to find out what their occupations were
11 October 1915 - Derby Scheme - voluntary enlistment/semi conscripted, war pension introduced.
27 January 1916 - Military Service Act - All British males between the ages of 19 to 40 (on August 1915), unmarried or widower (on 2 November 1915) were conscripted. This came into force on 2 March 1916. Most males were conscripted to the army unless they expressed a preference for the navy.
1 September 1916 - Training Reserves formed.
25 May 1916 - Military Service Act extended to include married men
10 April 1918 - extension of Military Service Act, age of conscription lowered to 18.
The site has so much information in it that it's probably better to dip into it at a leisurely, timed pace than try to look at it all at once. You will find general information about WWI, the army and more specific information about army units, battles, maps, finding service records - to name but a few. It helps that the site is split into three main parts - Soldiers, The British Army and The Great War. Thanks to Chris Baker for giving us this wonderful resource.
My grandfather was in the Royal Naval Division which later became the 63rd RND. He was in the 189th Brigade. While the RND was originally under the command of the admiralty it transferred to the army in 1916 and I was able to find useful information at the site.