Your next post will contain differences from your original. We'll call that 1.0.2 and so on and so on and so on.
Version control is a method for storing and tracking different versions of files for applications. When files are properly labelled and versioned <cough!>, it assists testers in verifying changes from one version to the next. There are many applications out there that can serve as a version control system ranging from CVS, VSS to ClearCase.
It also tends to be a full-time job to manage once your development group grows beyond 5-10 people.
A good configuration manager is worth his/her weight in gold in my opinion.
Actual quote from an email I received from a product manager:
"I want to tell all our customers to go to it (site) on Monday next week. I especially want to make sure the forums work right. Can you test every feature you can find?"
Think of a version as a slice across time for the code. Each code module will have different "revisions" within its line (as it moves forward in time, a vertical line). Each "version" is a thread across the particular revision of the modules of the entire code itself (a horizontal line).
You do this so you can keep track of the differences across the system for one build/release to the next. Let's you know what comprised that particular instance of the system.