(1) Refers to the "actual data" in a packet or file minus all headers attached for transport and minus all descriptive meta-data. In a network packet, headers are appended to the payload for transport and then discarded at their destination. In a key-length-value structure, the key and length are descriptive data about the value (the payload). See protocol stack.
(2) In the analysis of malicious software such as worms, viruses and Trojans, it refers to the software‘s harmful results. Examples of payloads include data destruction, messages with insulting text or spurious e-mail messages sent to a large number of people.

from http://www.pcmag.com/encyclopedia/term/48909/payload

On the Internet, a payload is either:

1) The essential data that is being carried within a packet or other transmission unit. The payload does not include the "overhead" data required to get the packet to its destination. Note that what constitutes the payload may depend on the point-of-view. To a communications layer that needs some of the overhead data to do its job, the payload is sometimes considered to include the part of the overhead data that this layer handles. However, in more general usage, the payload is the bits that get delivered to the end user at the destination.

2) The eventual effect of a software virus that has been delivered to a user‘s computer.

from http://searchsecurity.techtarget.com/definition/payload