Column widths, margins etc. The size of a typeface is always specified in points. Margins, column widths and so on can be specified in millimetres, inches or picas. One pica is 12 points. However, unless you're preparing a specification for someone else to follow, you can choose to set out margins, column widths and so on in whatever system you prefer to use.
Millimetres seem to be increasingly common.
Leading It is rare for a typeface to be designed with characters that occupy the entire height of the body. There will therefore always be a gap between two lines of type in the same face. However, this gap can be extended. In metal typesetting, the printer adds narrow strips of lead between the rows of type so as to increase the gap. This text which you are reading now has no additional leading and is therefore said to be 'set solid'.