Typically, a babysitter is different from a nanny as they would work for you in the evening. Generally a babysitter perhaps less experienced than a nanny as usually the children would be asleep by the time they arrive. However, many career nannies also babysit in the evenings to make up more hours.
However, the children do not need to be in bed for you to book a babysitter! You can book them whenever you need them! Usually they work on an ad-hoc basis - some parents like to interview and select a 'pool' of babysitters to call on for some cover or for when they want an evening out.
Step one → fill in the form
Step two → make payment (once you submit form)
Step three → receive applications
Step four → hire help!