Heuristic scanning was the next evolutionary step in av scanning after signature scanning. It essentially scans for malicious activity using a known ruleset of malicious behaviour. It's also known as behavioural scanning and can be very effective. For example a packed file that modifies registry keys and downloads another executable could be classed as malicious. Google has plenty of presentable examples. Be sure to highlight the pros and cons!

Pros: improvement on signatures
        Effective against unknown threats
        Effective against polymorphic (mutating) malware

Cons: false positives/negatives
         Requires user interaction
         Etc etc....

By no means a complete list, but enough to get you started...

Cheers,
n1p