The development cycle and the release/deployment cycle were earlier different and were managed by 2 different sets of folks. For eg, the dev team produced code and the sysadmin team upgraded/deployed it and managed the servers (operations) etc. Dev-ops is a movement, where these both are considered together so that the entire chain from dev to deployment happens seamlessly with automation . This is becoming possible, where there are frequent releases (following agile methodology) and also with virtualization technologies where the infrastructure (machines) can be managed with code, making automation easy.
In an nutshell, the development and operations coming together by way of automation.