There are no simple 'definitions' as such...it is a very complex and multifacited subject which to oversimplify would not explain or answer or do it or you justice...it is a multitude of airframe and system (electrical, hydraulic, avionics, FCS) integrity inspections/checks/recalibrations, lubrication, cleaning etc. etc.
Get hold of the appropriate aircraft maintenance schedule which tells you the manufacturers recommended tasks and check periods in detail...however be aware that a C Check input will also include additional Corrosion Prevention and Control Programme (CPCP), Structural Inspection Programme (SIP) and Airworthiness Directives (AD) which can make up 2/3rds of the check...ask to see a 'Check Pack' for a C Check input which will show all such tasks would be a good example...speak to your local friendly heavy maintenance people
...and without getting too involved you should be aware that operators will define their own Maintenance Schedule tailored to their operating/utilisation requirement which although accomodating the basic manufacturers recommendation may well alter the phasing or periodicity of check inputs considerably - in some cases doing away with a C Check altogether...then we have MSG 2 and MSG 3
