Inks are inks. They're intensely colored, translucent, and dry glossy. Washing and glazing are two
uses for inks, for which
GW offers premixed formulas ("Shade" being the new term for "Wash") that contain additives meant to aid in their respective tasks. Glazes are like somewhat dilute inks, applied generally to evenly tint a surface, changing the hue of underlying paint or increasing color saturation. Washes contain acrylic medium and a surfactant, which causes them to pool more deeply in recesses, resulting in a shading effect (due to the higher volume of pigment deposited).
Since the coloring agent is still ink, there is some potential crossover in their uses. For example, I use
GW's now defunct Thrakka Green wash as a glaze on Ork skin (as well as a wash, but that step comes earlier) by applying it sparingly enough to avoid pooling. The even coat allows it to function as a dilute glaze, slightly increasing the intensity of the green and softening visible layer transitions, but without undoing prior highlighting.
By getting yourself some inks, you're enabling yourself to use a single bottle for a greater range of tasks, at the expense of increased preparation (and, if you don't already have them, the acquisition of additives). If you're a frequent wash/shade user, mixing up your own (LBursley from APJ has a video on the topic that is usually the first place interested parties are sent - edit:
here's the Dakka thread with both video and text) will actually save you a decent amount of money, in the long run, but acquiring the mediums takes a bit of initial investment (they'll last you a long time, however). If that sound like too much hassle, Vallejo
does offer some premixed washes (I have the Game Color Flesh Wash, which behaves more like an ink than
GW's offerings, but is nonetheless effective), as do other companies (ArmyPainter's Dark Tone and Strong Tone are being heralded as the successors of Badab Black and Devlan Mud, respectively).
[edit:] Ha! Ninja'd, but glad I wasn't the only one who couldn't think of much to say about inks.