Google-search – What does “tbm” mean in Google Search

{google:baseURL} is defined in the source as http://www.google.com/. {google:RLZ} has to do with RLZ. The other two seem to have to do with what you type in the address bar and what you actually pick for Chrome to search on.

One of the reasons is so they can do click tracking on what link you clicked, on their search results page. This allows them to detect and optimize their search results.

For example if they noticed everyone that searches for "batman" only ever selects the 2, 3, 6, 7 links, they could remove the 1, 4, 5 links as they are obviously bad results for this search word.

Also you should take a look at Google's search history. I can tell you are logged in from this link and that your search history is being logged. This will give you a good indication of the information that Google collects when you click this link.