Who's tried it? I find it to be incredibly useful. Basically, with Ajax, you can reload a part of the page (as opposed to being forced to reload the entire page) after performing some other operation, perhaps with a database.
For example, imagine you have a web page which contains a form in which you can input a number. The form is then meant to compare the number you input to a number contained in some database, and then display either "It matches" or "it doesn't match" on the webpage. Now imagine that there is something else going on in the page, a video perhaps, which the user would not want to have reload when submitting the form. With ajax, you can send the value to a php file which can then get the value from the database, compare the two, and return the appropriate string of text that javascript could then dynamically update on the page without any sort of reload.
Check out a tutorial on ajax here.
For example, imagine you have a web page which contains a form in which you can input a number. The form is then meant to compare the number you input to a number contained in some database, and then display either "It matches" or "it doesn't match" on the webpage. Now imagine that there is something else going on in the page, a video perhaps, which the user would not want to have reload when submitting the form. With ajax, you can send the value to a php file which can then get the value from the database, compare the two, and return the appropriate string of text that javascript could then dynamically update on the page without any sort of reload.
Check out a tutorial on ajax here.