Indexing and crawling are two different but related concepts that are important to understand for any website owner and for anyone interested in search engine optimisation.
When Google sends its spider (Googlebot Smartphone) to crawl your website it starts to visit pages to download and analyse your content and website code. Google then decides how much of your site content is worthy to get indexed. Google will come back and crawl your site at the time interval that it determines.
This is a fairly large topic. A good starting point is creating a Google Webmaster Tools account and reading Google’s guidelines for webmasters.