What is Crew Resource management (CRM)?
CRM is about making use of all available resources to safely conduct a flight.  Pilots these days (even single pilot ops) have a wealth of resources available to them.  Anything you can see and anyone you can talk to is a resource, and CRM is about making efficient use of those resources.

Flying a light single you will have a subset of these resources:


Cockpit displays
Charts
Checklists
ATC
FSS
Flight Watch (EFAS)
Live Weather downlinks (e.g. XM satellite weather)


On a large transport aircraft these change a bit:


Cockpit displays
Charts
Checklists
ATC
FSS
EFAS
Onboard RADAR
The QRH (quick reference handbook)
The FOM (flight operations manual)
The other pilot(s)
The cabin crew
Passengers (Doctor in a medical emergency?  People helping in an evac?)
Dispatch
Medlink
Company Ops
For ground ops:

Fuellers
Ramp personell
Gate agents



More and more emphasis is placed on managing these resources as you move up the chain.  By the time you are taking your first 121 upgrade checkride, it is more about judgement and CRM than it is about the flying.