Covert channels and side channels are two types of information leakage channels. A covert channel uses mechanisms that are not intended for communications, e.g., writing and checking if a file is locked to convey a "1" or "0".
In a covert channel, an insider process leaks information to an outsider process not normally allowed to access that information. The insider (sending) process could be a Trojan horse program previously inserted stealthily into the computer. An outsider (receiver) process need only to be an unpriviledged process.
 Covert and Side Channels due to Processor Architecture, by Zhenghong Wang and Ruby B. Lee, in ACSAC 2006