Intellisense is Visual Studio's version of autocomplete. The DB is where it stores all of the project information that is used to pop up options. For instance, I use the Apache XML library in my code. If I have DOMElement * element declared in my code I can get a list of all members simply by typing element->. I can then select each member and get a popup of the embedded comments for that member. It basically indexes everything about your project so that information is at your fingertips.