Just wondering. I thought shinobi meant ninja, but sometimes they say ninja, and sometimes they say shinobi. Is there any difference between the usages?

shinobi to describe a person who provided special services. for an important person in an old japanese kingdom. in other words for example: the nara period.(624-711) the shinobi were used for any special tasks such as delivering important messages, performing special tasks, such as killing a group of others whom may look suspicious to his master. etc.