It all amounts to changing colour, but I came to the conclusion that there is a difference. Intarsia is worked by using bobbins or butterflies of yarn which you pick up as and when you need to change colour and this means that the pattern can be seen clearly on both sides of the work (although I think you can still tell which is the wrong side). You can see how it works here.
Tapestry crochet is where you join in your new colour but, while working your new colour, you enclose the original colour in each stitch so that it doesn't get left behind and is ready to pick up when you need to change again (if I'm still getting it wrong please tell me). The wrong side of the work with this method is not as neat. There is a good demonstration of how it works here and this is the method I used in my pincushion for the pink centre blocks.
It didn't take long to work and I'm quite pleased with the result but it wasn't all plain sailing: I had worked my pink yarn into the second white block before I realised that I didn't need to - doh!
Hope all is well out there ...