One of the major tools in your arsenal of home improvement.
A putty knife is one of those friendly kinds of knives that are not sharp. They do have pointy corners, however, so you should handle them with a modicum of care. They have a handle, much like other knives, and they typically have a few inches of metal sticking up out of it. The 'blade' can be from 1 inch to 6 inches (and up for certain models) wide. The 'blade' is perpendicular to the axis of the handle, unlike most knives. Though they typically don't have a keen edge, it is important that the business end of a putty knife is smooth and straight.
Putty knives may be used to shape and trim putty, mainly in the glazing of glass windows (where you put in panes of glass into old-style windows that are then held in place by glazing putty), but putty knives can be used in oh so many ways. One of the obvious uses is in the application of spackle or joint compound onto walls. They are also useful for light scraping.
Putty knives are somewhat disposable. Too much scraping scratches the 'blade', and scratched putty knives no longer apply spackle to walls smoothly. They are made of cheap metal, so they rust and bend quite easily. They are cheap, however, so they can be replaced easily.
They sell plastic putty knives, but they do not work as well as metal ones. Plastic tends to bend, so they don't apply putty to walls very smoothly. They don't scrape very well at all.