First off,
this article would be a good read.
This is a good one too. I hope those make your understanding of memory management in OS X a little more clear.
In that picture, you have ~30MB free, ~200MB wired (being used by OS X), ~1GB active (in use by applications) and ~530MB inactive (not really in use but will be used if a previously open application wants it's memory back). If you run out of free memory, memory from the inactive figure will be used.
Inactive memory, while looking like it consumes more memory is quite helpful. For instance, let's do an experiment. Open an application that you haven't opened since you last boot. Make a mental note of how quickly it opens. Now, quit it completely and re-open it. Notice how much faster it opens? To quote the second article in its discussion about inactive memory: "...the idea being that if you opened an application you are somewhat likely to do so again and if the memory is still labeled, the application can start very quickly".