In a nutshell, the campaign arose out of the need to contain Russian expansionism and to protect Turkey from the threat of having its interests undermined - the Turkish-controlled Bosphorus being the only exit from the Black Sea for Russia's fleet. An Anglo-French expedition was mounted to assist Turkey to this end. The (then) Kingdom of Sardinia also provided a token force.
Obviously, we had forgotten the first rule of war: don't get involved in a land war in Asia. Fortunately no Sicilians were involved.
Needless to say, the extreme suffering & hardship suffered by the British & French armies highlighted just how ill-prepared they were for such an arduous campaign; the equipment & methods of fighting having changed little since Waterloo. A few senior British officers (who were veterans of the Napoleonic wars) still referred to the French as 'the enemy', despite them being allies - though I think they might have been on to something.
Notable "firsts" were:
- First photographs from the front lines
- War correspondents sending reports by telegraph ... bloody journos!
- Tactical use of railways
- Florence Nightingale
- Indirect artillery fire.
It also saw the British infantryman armed with a muzzle loading .577 inch Enfield 2nd Pattern (1853) which finally gave the 19th century squaddie a weapon as good (accuracy & range) as the medieval Longbow.
A fuller version can be found on this wiki page.