A liaison in Second Life is a Linden Lab employee that will answer a resident's questions and may do other tasks such as clean up junk or handle disputes. Liaisons may or may not know the answers to technical questions but will be able to point you in the right direction. A liaison is usually not an SL developer Linden. You can tell if a Linden is a liaison by looking in their profile.
Most liaisons are part-time employees of Linden Lab and most of them are regular Second Life residents when not on-duty.