Explanation of ICMP message types

Network error reporting

Destination Unreachable, Echo Reply: Sent by a router when it cannot deliver an IP datagram. A datagram is the unit of data, or packet, transmitted in a TCP/IP network.
Source Quench: Sent by a host or router if it is receiving data too quickly for it to handle. The message is a request that the source reduce its rate of datagram transmission.
Time Exceeded: Sent by a router if the datagram has reached the maximum limit of routers through which it can travel.
Parameter Problem: Sent by a router if a problem occurs during the transmission of a datagram such that it cannot complete processing. One potential source of such a problem is invalid datagram header.

Echo Request, Echo Reply: Used to test destination accessibility and status. A host sends an Echo Request and listens for a corresponding Echo Reply. This is most commonly done using the ping command.

Redirect: Sent by a router if it receives a datagram that should have been sent to a different router. The message contains the address to which the source should direct future datagrams. This is used to optimize the routing of network traffic.
Router Advertisement, Router Solicitation: Allow hosts to discover the existence of routers. Routers periodically broadcast their IP addresses via Router Advertisement messages. Hosts may also request a router address by broadcasting a Router Solicitation message to which a router will reply with a Router Advertisement.
Timestamp Request, Timestamp Reply: Used to synchronize the clocks between hosts and to estimate transit time.
Information Request, Information Reply: Obsolete. These messages were used earlier by hosts to determine their inter-network addresses, but are now considered outdated and should not be used.
Address Mask Request, Address Mask Reply: Used to find the mask of the subnet (i.e. what address bits define the network). A host sends an Address Mask Request to a router and receives an Address Mask Reply in return.