Hi,
The technical definitions of coldStart and warmStart are:
A coldStart trap signifies that the sending
protocol entity is reinitializing itself such
that the agent's configuration or the protocol
entity implementation may be altered."
"A warmStart trap signifies that the SNMPv2 entity, acting in an agent role, is reinitializing itself such that its configuration is unaltered."
The key difference in this reinitialization (reload or reboot) is altered versus unaltered.
However basically they are:
coldStart -- The snmp device performs a power on
warmStart -- The snmp device performs a software reload or ipl