A framework is a basic (usually abstract) conceptual structure which you can use as a “scratch” for your web-projects. For instance, instead of defining global reset, consistent baseline, typographic rules or basic styles for forms over and over again — every time you work on a new project — you can prepare a default-style once and reuse it in all your future projects. This is what you call a CSS Framework.
CSS frameworks don’t have to be complex or large, they may contain a set of simple CSS-styles such as
typography.css for basic typographic rules,
grid.css for grid-based layouts or
layout.css for general layouts,
form.css for basic form styling,
general.css for further general rules

and so on. In your code segmentation you can also go further, for instance: structure, typography, design presentation, specialist sections (e.g. menus, navigation), print, mobile web, tweaks (mostly old style browser hacks), browser specific workarounds (via IE conditional statement). “On the whole code segmentation in frameworks is handy to work with, but it can add some real load to a server with the extra http request per page view.” [Treading Lightly With CSS Frameworks, by Gary Barber]
“[Framework is] a set of tools, libraries, conventions, and best practices that attempt to abstract routine tasks into generic modules that can be reused. The goal here is to allow the designer or developer to focus on tasks that are unique to a given project, rather than reinventing the wheel each time around.” [Framework For Designers, by Jeff Croft]