A pastebin is a site that allows you to share text with other, most commonly source code, but could be anything else as well (like prose or poetry - e.g. you are writing a novel or poem and want to share what you write). They are useful if you do not have a web site to post what you have written and want to share it to get any feedback (like spotting any bugs or spelling mistakes).
They allow you to post without registering or paying and the pastes normally expire after a set period (e.g after a month) so as to reclaim any lost disk space for the host. Source Code highlighting is often a feature of many as well. Some of them offer more than this, like comparison (e.g. diff's), password protection / encryption and notifications (to alert you of new pastes, or replies to pastes - normally through a chat system like IRC). The open nature of them could render them susceptible to spam though.
The following is a list of pastebin's I am aware of. All offer syntax highlighting.
- pastebin.com - possibly the first site to do this, you can view recent pastes and search them
- pastebin.ca - more features than pastebin.com: encryption, tags and description
- nopaste.info - description, name, tab width
- LodgeIt! - very simple interface, powered by jQuery, reply to pastes, view diff's, remote queries (XML-RPC). You can't see recent pastes or perform a search though (although this probably mitigates spam somewhat)
- pastie - separate code into sections, mark as private, select a theme when viewing pastes
- dpaste - associate a title and your name / email