ORBAT - or ORder of BATtle - is a term used to refer to the way a military unit is organised. The ORBAT for the British Army is a product of hundreds of years of tradition and excellence - and pretty incomprehensible to those accustomed to militaries organised along more logical lines.
Types of ORBAT
There are a number of different ways to approach the subject:
Administrative ORBATs are used to manage affairs whilst in Barracks. For instance, the Royal Logistics Corps. It's not a warfighting unit made out of several Divisions, it's a capbadge where all sort of loggies hang out. These administrative orbats don't necessarily make sense - the Parachute Regiment, formed as it is from a number of Infantry Battalions should really be called a Brigade - but then again, admin is for hats.
Special To Task
Point to Note: There are fixed all-arms formations in existence but these never seem to go to war in their defined forms.
Other Key Terminology
Don't forget that above about Battalion level, there are many ways to organise matters. The old Warsaw Pact Army was formed from Divisions, each was made up of Regiments, each of which was made up of Battalions. This faced a NATO largely made up of Corps, formed from Brigades which in turn were made from Battalions. Although not always. And as mentioned above we called tank Battalions Armoured Regiments.