Malware Defender is what is known as an HIPS (Host Intrusion Prevention System) which basically means it protects you from all kinds of malware and spyware that may try to hijack your system.
The key advantage of Malware Defender, however, is that it is very low on resources despite featuring a realtime protection system, process manager, kernel module manager, hooks detector, autostart application manager, file explorer and a registry Editor. Combined, these protect your from viruses, worms, Trojans, adware, spyware, keyloggers and rootkits.
Malware Detector doesn't seem to be the most intuitive program to use. The reason it is so lightweight is that it's very stripped down, meaning that the results are reduced to nothing more than a crude list of its functions and status. It does however run incredibly discreetly which makes you wonder whether it's really having an effect but it seems to pick up everything that Adaware, etc. finds.
Malware Detector is probably not a great starting app for beginners but if you need a lightweight malware scanner, it's a decent option as long as you know what you're doing.