Basically the spiders need to access your home page and crawl whatever links it finds there. Crawl is depending on a number of factors, but let’s assume you have an easily accessible site rich with content and that the spiders have crawled every page. It's called crawling.
The next step is for them to decide whether or not to put the information they find into their index. If they find it unique and well enough, then decide to index all of it. Great! At this point, you can say that your pages have been indexed, called indexing.