There are five different field types that a field can be used as:

Text:

This will simply display the data as text, and you can optionally use the Format function to change the format if the original database type is a number.

Number:

This displays the data the same way as text would, and can use the Format function on this data type as well.

What's the difference between Text and Number? Both display on the report the same way and use the same Format functionality, the difference between the two comes into play with Band Scripts; Text returns as a string, Number returns as a double.

Image:

The Image field type works by converting your image stored in your database in a blob (Base64 Encoded) back to an image format. There is no way of using stored images in fields without the use of Band Scripts currently.

Barcode:

Once you select the Barcode field type, you can select what kind of barcode you want to generate. It will use the data field as the string for the barcode.

Checkbox:

If your data is boolean, the Checkbox will display it as True or False. The checkbox has options for the unicode characters to use as the checkbox, which you can replace with anything you want to.