NaBloPoMo stands for National Blog Posting Month. The goal of NaBloPoMo is for participants to write a post a day for an entire month.
Although November is the official month for NaBloPoMo (with prizes and everything!) people are invited to take up the challenge of writing a post a day during any month. To help inspire bloggers, NaBloPoMo assigns a theme for each month, and also offers daily writing prompts, but these are optional. Bloggers are not required to write on a specific month's theme or a specific day's writing prompt. Any post about anything at all counts.
NaBloPoMo's theme for March 2011 (for those who'd like to write on it always, sometimes, or never) is "In a word."
* Note: NaBloPoMo moved in 2011. More information here.