Weight refers to the blackness or heaviness of a character, and generally ranges in scale from light to bold. Weights from one typeface to another can vary considerably, but they have relative meaning within a typeface family. Some typeface families have only a few weights. Bernhard Modern, for example, has only Roman and Bold, and Clarendon has three weights—Light, Roman and Bold. Other typeface families, such as Helvetica Neue and Interstate, have a huge range. Helvetica Neue has eight weights, from Ultra Light to Black.
In typography, a ligature is a single glyph made up of more than one character. Ligatures can be functional or ornamental. A good example of a functional ligature is the combination of the letters f and i. In many typefaces the dot of the i crashes into the arch of the f, and the ligature resolves the overlap of letters and improves letter spacing. The typefaces shown here are Adobe Caslon, Eames Century Modern and Adobe Garamond Pro: the second two also contain the ff and ffi ligatures in their glyph set.
A true italic is an angled typeface designed to accompany its roman counterpart. It is designed with its own features and character widths, and is often quite calligraphic in appearance. An oblique is usually a slanted version of the roman face, often with very little change to the design of the letter shape. Of course, as with all things, there are exceptions to the rule: Helvetica and Optima are two examples where the oblique is actually an italic. Programs like InDesign make it possible to slant any typeface, which can come in handy if the type family does not contain an italic weight, but where an italic has been drawn, it is usually much more complementary to the roman face it is matched with.
The word aperture comes from the Latin apertura, meaning opening, so it follows that, in typography, aperture would refer to an opening in a letter from. Specifically, it is the opening to the partially enclosed negative space (or counter) in characters such as a, c, e, n and s. Apertures can be small or large depending on the typeface. The lower case e of Berkeley Oldstyle Book has a large aperture due to the angle of the horizontal bar, but the e of Bembo Bold has a relatively small aperture. Vectora’s a has a tiny aperture, and while Serifa Light’s is larger, it is still small compared with that of Today Extra Light.