Spam is a scourge for all websites, so webmasters have to take measures to ensure that thier sites are protected. One of the best ways of protecting a site is to create a task on the signup/commenting forms, that only a human can do with reasonable accuracy. CAPTCHAs ("Completely Automated Public Turing test to tell Computers and Humans Apart") as shown below ask you to read some distorted text, which the average computer finds very hard to do.

You can use the control buttons in the captcha to help you as follows.

Refresh Button - If you're having trouble working out the words to enter, you can request a new set of words, without having to submit your form by pressing the refresh button.

Audio Button - Alternatively if you have trouble with your vision, you can use the audio CAPTCHA, pressing the audio button will playback a set of words for you to type in.

The words you enter need not have the same uppercase/lowercase as the images shown, the system ignores case, likewise it only uses the characters a-z and 0-9, so you dont have to enter any odd characters, accents etc.