"Webspam" refers to pages that try to trick Google into ranking them highly. Before you file a webspam report, see if the page might have a different problem:

Paid links Copyright and other legal issues
This page is selling or buying links. This page should be removed under applicable law.
Objectionable content Personal/private
This page is inappropriate. This page discloses private information.
Malware Phishing
This page is infected. This page is trying to get sensitive information.
Other Google products Rich Snippets
This page abuses Google products other than Search, e.g., AdSense, Google Maps, etc. This page doesn't comply with Google's rich snippets guidelines.
Something else is wrong
This page has other, non-webspam related issues.
This page is really webspam.