Nessus and Acunetix do two quite different things. Nessus will not find XSS or SQLi issues in a web app for example - it will find problems where for example a reported Apache version is out of date and has a security vuln, and other OS level issues. It will not find problems with poorly written web code, unless there is a specific widely publicised flaw in a well known product.
Backtrack has a ream of tools for attacking / pen testing sites and servers. (for example Nikto but you do need to know what you are doing to use them.
Acunetix is a more accessible product , but of course you pay for that privilege. If your time is more important than your money, I would say get both Nessus and Acunetix.
If money is an issue, then utilising the array of tools in Backtrack is probably the best solution, but it's intimidating and will soak up a lot of your time.
Not sure if you're being sarcastic, but i'll answer you anyway. A vulnerability scanner is a piece of software that checks for rudimentary security flaws in a piece of software (desktop, or web application). It does so by using one of several techniques, namely: dynamic or static analysis. As the name suggests, dynamic analysis is the process by which you test the software as it runs. Static analysis requires access to the software code, and looks at the actual logic of the software.
Examples: http://cirt.net/nikto2 http://www.tenable.com/products/nessus