Networking Blog

When Access Control List Logging is activated on your Cisco device, you will see log entries like that in syslog:

Jun 14 09:49:13 RTR-1 41716: Jun 14 09:49:12.148 MEZ: %SEC-6-IPACCESSLOGP: list 120 denied udp x.x.x.x(1670) -> x.x.x.x(4808), 1 packet
Jun 16 22:14:09 RTR-1 42271: Jun 16 22:14:08.847 MEZ: %SEC-6-IPACCESSLOGDP: list 125 denied icmp x.x.x.x -> x.x.x.x (0/0), [...]

syslog is the most common protcol for transmitting system messages over an IP network.
It is described at length in RFC 3164.
At the bottom of the post, you can find the meanings of syslog facility and severity (also known as priority)
Layout of a syslog message

Name of program
Facility
Priority/Severity
Message

Note: Not all parts are visible in the logfile(s), but [...]