The table below is an attempt to summarise the relationships between and the sizes of various military units. These sizes (and sometimes names) will vary not only from country to country, but also by the nature of the unit and its particular circumstances. The numbers given for men in each unit is more representative of infantry units than armoured units.

Unit Name Consists of [1]: Approx Number of men: Commanded
by:
Army 2 or more Corps 100,000 to 150,000 Field Marshal or General
Corps 2 or more Divisions 25,000 to 50,000 General or Lt. Gen.
Division 3 or more Brigades or Regiments 10,000 to 15,000 Lt. Gen or Maj. Gen.
Brigade 3 or more Battalions 1500 to 3500 Maj. Gen, Brigadier or Col.
Regiment[2] 2 or more Battalions 1000 to 2000 Col.
Battalion 4 or more Companies 400 to 1000 Lt. Col.
Company 2 or more Platoons 100 to 250 Captain or Maj
Platoon (Troop) 2 or more Squads 16 to 50 1st Lt.
Squad 2 or more Sections 8 to 24 Sgt.
Section   4 to 12 Sgt.
Notes:
[1] This column indicates what usually makes up the core of the unit - with larger formations there will be more supporting units as well such as units for headquarters, admin, signals, air-defence etc etc which will increase the overall number of men involved.
[2] Regiment is not usually a deployed unit as a whole, but is more of an "administrative family" or permanent label of a group's identity. So units from a particular regiment may be spread around within an army.
 Sources:
The information above was cobbled together from many sources, encyclopedia, dictionaries, newsgroups and anywhere else I could find.