I can't answer for Alwil and a proper answer would take many pages, but....
Compared to the standard technique of determining that a file is an exact match with an already recognized and classified threat, heuristic detection is an 'educated guess', a simpler, quicker way of determining that a previously unseen file is likely to be dangerous based only on its resemblance to other files. It's a pragmatic compromise aimed to be of benefit in reducing AV overhead and/or catching previously unseen threats and is prone to 'collateral damage', i.e. false positives. This is at least partly why avast! has heuristic scanning email but not for the Standard Shield or other providers.
Instead, avast! relies on extremely fast definition updates and more recently, generic signatures e.g. 'trojan.gen', whereby the 'educated guess' is made by those analysing and compiling the definition update rather than by the program itself in real-time.
(Note: The latter is my own supposition so please, someone from Alwil, correct/qualify as necessary.)