The report button, shown on all links and comments, is an anonymous way for redditors to tell community moderators that something is spam or otherwise violates reddit or community rules. For example, pornographic content submitted to a non-adult community, or a .PDF posted to r/videos.
If the reason for reporting is time-sensitive or non-obvious, please explain your reasoning by sending a message to the moderators.
Once something is reported, it will be reviewed by a moderator of the community. The more people that report an issue, the more likely some action will be taken. If enough people report something, Reddit site administrators (employees) will be informed.
You can also report spammers by submitting the offending user's profile page to the /r/spam community. Reporting spam is the single most important thing you can do to help Reddit.