Their use is dependent on the software, but in general, they can be used to write or draw on with a stylus or a finger if it has a capacitive screen. The software can also be installed that allows images to be projected onto the IWB. This is useful for displaying images or videos relating to what's being taught and used as part of a presentation.
Smart Interactive whiteboards can also be helpful for student presentations. A student can use the board to write notes and present them to the class as if they were projected onto a screen. The student can then "draw" over their notes with their finger, which improves learning by involving tactile senses more actively in the learning process.
An interactive whiteboard is unlike a traditional whiteboard in that it's nonporous; it doesn't allow ink or wet-erase marker to penetrate through the material. So, you can use a dry-erase marker on it as you would any other whiteboard.