I have seen quite a few great illustrations placed as headers or footers, for example, see this or this. I was wondering which tools are used to make such illustrations. Can I create this kind of images in Adobe Photoshop or Adobe Illustrator? Are they made by using the pen tool?. I'm asking this because I have read some comments complaining about the difficulty to create a nice drawing in Photoshop.
I've also seen discussions related to vector drawings as opposed (I guess) to bitmaps drawings. What is the difference between each other in relation to web design?
Thanks in advance.