The new sites are indexed by Google with the help of “Bots”. A Bot is piece of software from a search Engine that is built to go through every page of a site, then categorize it, and place it to the database.
Google has three well known bots:
* The Adsense bot
* The Freshbot
* The DeepCrawl.
The Freshbot crawls the most popular pages on your website, doesn't matter if that is one page or thousands. A typical site should expect to have a Freshbot visit every 1 to 14 days, depending on how popular those pages are. Freshbot places those crawled links into a database so that when the DeepCrawl occurs, it has a reference.
Once a month, the DeepCrawl bot visits your site and goes over all the links found by the Freshbot. This is the reason why it can take up to a month for your entire site to be indexed in Google - even with the addition of a Google Sitemap.