A hashtag is a word combined with a hash (#) that allows you to categorize your posts. For example, if you write a post about NGOs, putting hashtag #nonprofit, followers will automatically know what you are talking about.

It's also usefull for researches. On Twitter, when you type a "global change" in the browser, all the posts containing this hashtag will be listed. Hashtags allow your organization to participate and inspire conversations (and in the process get new followers).

Moreover, there are several hashtags commonly used by NGOs.
- #Nonprofit: Occasionally, you can replace the word “nonprofit” with “#nonprofit” in a Tweet, or tag it on to the end of Tweet that mentions a nonprofit or nonprofit trend.
- #nptech: This hashtag stands for nonprofit technology and can be used in any Tweets relevant to how nonprofits are use technology and social media.
- #[event] and [campaign] These are the most creative and useful hashtags being used by nonprofits on Twitter. If your organization is using Twitter to promote an event or campaign, make sure your decide on a #hashtag and consistently use it while your are promoting the event and/or campaign.
- #volunteer[s]: This hashtags can be useful to send out a call for volunteers, to thank volunteers, or to promote the work of your volunteer community.

And many more exist! Vous can also create some. Just check first: http://twitter.com/hashtags to know if the one you want to create, already exists and then go on http://www.hashtags.org//

A word of caution though… use them strategically. If you use hashtags too often, then you dilute their usefulnesses by fragmenting the conversation.