I believe it varies by region. In this part of the world (Israel), a kebab is ground meat, with binding agents (breadcrumbs, egg, I dunno) and spices on a stick. Shipudim is anything on a stick, and shishlik is generaly chicken breast on a stick. And schwarma is pressed-together meat (turkey, usually, lamb, traditionally, and chicken and cow are also available)on the above-described rotisserie arrangement. Not ground, though, and not with breadcrumbs or anything like that.

There is no such thing as a shish kebab arond here, and meat rarely comes skewered together with vegetables.

I believe kebab is derived from an Arabic root meaning "round."