Alt text is the description of an image that allows all users but specifically those using screen reading software to gain the same information from an image in text format that one would by seeing it. All images in digital materials should have informative alt text, however images that are strictly for decoration only require a single empty space so that screen readers will skip over it. 

It is not enough that there is alt text for images, it must be useful to those using screen readers and other assistive technology. With that in mind there are best practices for when writing alt text.

Alt text should be a description not a file name

Descriptions of images allow useful information to be shared. A filename is often not useful to users. 

Provide enough information in your alt text to be useful

Keep in mind your audience, thinking about what information users will need from the image.

Stay away from any irrelevant details that users won't need

Too much detail becomes cumbersome and convoluted to those using assistive technology. If it isn't relevant to those who are sighted it shouldn't be added to an alt text.

Keep descriptions short and succinct 

Avoid lengthy descriptions where possible and be direct in those descriptions.

Use appropriate and relevant language

Keep information at a vocabulary level the user will understand.

If the image is of text then the alt text should be that text in the image

For example if an image is just text, like a "home" button the alt text should just stay "home".

Use punctuation as you normally would

Assistive technology will read alt text just as it would other text, no special punctuation or characters are necessary.

Don't use phrases like "image of" or "graphic of"

Unless it is important relevant that it is a photograph, painting, etc. leave out those phrases. It's usually apparent to a user that the alt text is for an image and any extra would be unnecessary .

Don't provide the same information that is already in text into alt text

Redundancy in alt text information can become confusing or convoluted to users.