There are many ways:
- Simplest is to just use a camera and take a picture of the whiteboard at critical stages
- A more advanced way would be to go fully digital, using a smartboard wall type of solution (ie. big hunking display)
- Cheaper solutions work by way of using the computer for the display, a normal projector, and special hardware (like pens) to detect where on the wall you're pointing or doing things
I've tried all three and by far I liked the full display the best, but it is quite expensive in some cases.
A good enough solution is the one that integrates with the computer.
The solution I tried had a suction cup rod that you hung up on the whiteboard, along one side, connected to a computer through a long USB cable.
Then you had special "pens" that were just a normal whiteboard pen with a holder around it. This holder had some kind of radio connection with the rod, and due to the length of the rod, the rod could quite accurately pinpoint the location of the pen on the whiteboard. It also made a clickety sound when you pressed the pen onto the whiteboard, so it knew when you were actually drawing with it.
This was then relayed back to the computer, so everything you drew would be captured, and could be replayed later, line by line if you wished. There was also an eraser that came with it, but it wasn't quite as accurate.
I don't remember the price of the whole setup, but apart from the normal computer, the rest came in under $1000.
The full digital smartboard I've tried was basically a 50" LCD with pens that communicated with the computer. This of course had features such as multiple pages (easily switch back and forth, bring up diagrams from earlier discussions, etc.), full replayability, could stream contents live to participants not in the same room, etc.
There's also setups that use a projector, typically mounted in the ceiling pointing down on the wall, and again communicating with a computer, using the projected display instead of a LCD display. Never tested this.
The cheapest solution is the digital camera way, and since most phones comes with adequate cameras these days you already have what you need.