Define Alt tag?

The alt attribute also called as alt tag are employed in XHTML and HTML documents in context of defining alternative text that is supposed to be rendered when the element can’t be rendered to which it is applied.
One great feature of alt tag is that it is readable to ‘screen reader’ which is a software by means of which a blind person can hear this.
In addition, it delivers alternative information for an image due to some specific reason a user can’t view it such as in case of slow connection and an error occurred in the src attribute.
For example, the HTML for this image will appear something like this:

According to the definition of Penn State university " ALT Tags are invisible descriptions of images which are read aloud to blind users on a screen reader.
Adding ALT text allows authors to include images, but still provide the content in an alternative text based format.
If no ALT tags are provided, then a screen reader would only be able to say "IMAGE" or perhaps provide a file name" My simple definition : When you place a mouse over a picture in a website and it is say " image " it means that the ALT Tag is empty.
Webmasters are suggested to fill the alt tag with a descriptive meaning Example : Empty alt tag will show in HTML as this : Look at the alt= you will find nothing and that's is when it is displayed For SEO : ALT Tags that teach ' Search Engine Bots' about the content of the website , better than being just left empty.
It counts as a higher SEO Score as the ALT Tags are filled.
Now it is more of a best practice Vs a General rule.
Google Penguin Updates : Other factors play major roles in SEO Score such as Fresh non stale and unique content.
HTML5 and CSS3 It also helped Search engine bots overcome many hurdles in order to learn about content of a page.
Tarek Hassan , Certified SEO http://www.hubac.com

alt tag used in html

The best way to describe it is.
There are two to thing which derive Alt Tag, First part its is used in HTML Coding in Webpages for a website.
In Term of SEO, Alt tag plays an important role in increasing the keywords density of the targeted keywords that you want to target on search like google, bing, yahoo etc.
I hope this will help you to remove the confusion you have in in your mind if you have any other question feel free to ask it.
Thanks Cheers

you add alt tag to the pictures on your website ,so the search engine can archiving your images and show the results when someone search for the keywords on the alt tag

Alt Tag - we use Alt Tag to define our Image.
In some cases if your server is too slow or your Image is too big and it is not uploaded to the server.
So the Alt Tag we use here it comes.
because in Alt Tag we use our important keyword or shot description.