The pica is a unit of length in the field of typography and uses the symbol pc.
There are 12 points in 1 pica. 1 pica represents approximately 1/6 of an inch.
Picas are expressed using numbers and a lower case p on software such as Adobe InDesign - for example 8p6 which would represent 8 picas and 6 points (or 8.5 picas).
The Pica is very useful in measuring and calculating the space needed to print letters and words on paper - for example when laying out books or magazine articles; the font lengths are quanitified by the number of characters per pica (cpp).
The caliber is a unit of length based on the inch (imperial units) and uses the symbol cl.
It represents the internal diameter of a gun barrel and therefore more often than not, the size of the ammunition or round that gun requires to fire.
It is not always represented or converted exactly in inches so a table or "well-known" conversions are often used to depict measurements / ammo types instead.