the named style of the text
(ex. Times, Courier)

fonts are roughly divided into families according to their style or structure

the major families are
...serif
...sans serif
...old style
...decorative
...script
...modern

physical size of the letter on the page/screen

size is usually measured in "points" so that 24 point is much larger than, say, 9 point

24 point courier
14 point verdana

the horizontal spacing between characters

character spacing is especially useful when working with alignment...i.e. spacing characters in one word to make it's width equal that of another word

the vertical space between lines of text ("single spaced, double spaced, etc.)

usually only important in block text (as in word processors)

if you're working visually (in Photoshop) you can avoid this by placing text lines in separate layers and positioning them that way