Hello!
1. I'll quote Matt Cutts on this "...just make sure that you have the words that you want to have on the page, make sure that they read naturally and you should be in pretty good shape." So, this tells us that there are no precise rules to stick to, only rough guidelines.
However, it's still better to have more than 100 words per page, especially on content pages (e.g. about page, features, blog posts, etc.), where it is wise to have a minimum of 300 words, but remember, only if it makes sense.
Here's a video by Matt Cutts on this topic...
http://www.youtube.com/watch?v=Rk4qgQdp2UA
2. Yes, search engines do ignore certain words, these are so-called "stop-words."
You can read about them here...