The English used in this article may not be easy for everybody to understand. (June 2012)
A shirt is a kind of cloth which is made to cover your upper body. In North America, shirts can mean T-shirts, polo shirts, etc. In Britain, a shirt has a collar, sleeves with cuffs and split in half with buttons on one side. (North Americans would call this a "dress shirt", a specific type of "collared shirt"). Some shirts have sleeves, that cover your arms. T-shirts are a type of shirt that has shorter sleeves. These sleeves cover the shoulders. Shirts that do not have sleeves are called "sleeveless".
The basic measurements are from XXXL (extra large) to XXS (extra small) used during industrial productions. More accurate measurements use the collar, which has indication from 37 cm (14 in) up to 50 cm. Only shirts made by tailors (people who make clothes as a living) use many measurements  for a shirt that fits the best.