Short answer: Google Dance - Major Google updates in the past, most don't cal them that anymore. Doorway - Creating a page specifically for search engine rankings and nothing else. Generally not a good tactic and if done poorly can get your site penalized or even banned.
3)deepbot and freshbot There are two types Googles webcrawler/spider that are deepbot and freshbot. The freshbot is constantly crawling the web and finding fresh data. It usually happends on high ranking or frequently updated pages. The deepbot indexes the entire web once or twice a month. One is a "deepcrawler" that indexes the entire web once or twice a month. Note that the freshbot's IP start with 64.* whereas deepbot's IP start with 216.*. However, there are no more deepbot these days. The freshbot doing both duties - fresh and deep at the same time.