An A-line bob is a hairstyle in which straight hair or lightly waved hair is cut short (a bob), but with the hair in front cut longer than the hair in back. The prototypical A-line has the downward 'point' of the bob come to an approximately balanced V point, with the point just brushing the shoulder, or cut to rest just before the shoulder. The long front locks are brushed back, and the back is trimmed to (approximately) match this angle.
Frequently, at least under the current fashion, A-line bobs are asymmetrical, with one side of the head having the locks cover the ear but not the face, with the other side falling more forward, often covering or nearly covering the eye (it is most often the left eye that is thus impinged on). Fashion pictures will often show the impinging lock all the way to the nose, or in some cases falling just past the mid-line to cover the tip of the nose.
In common usage, an A-line may have no A-line. As long as the hair is no longer than shoulder length, is long in the front, and at least a bit shorter in the back, you can call it an A-line bob. While terminology is fuzzy, these deconstructed bobs may also be called a chin bob or a blunt bob, while a 'proper' A-line might be referred to as 'sharp A-line'.
An A-line bob may sometimes be called a V-line bob (this is obviously the correct way to describe it, but is rarely used), or an inverted bob.