Firstly, there's data in transit, also called data in motion. These simply describe data on the move, perhaps traversing across the network between devices or even between storage media, actively moving between locations.

Then there's data in use. Data in this state is currently being accessed by a user, or processed by a CPU. When data is accessed or used, it's pulled from the hard drive and temporarily stored in RAM, which is much faster than the hard drive (particularly mechanical drives) and stored there for as long as the user accesses it and there is power to the device.

When data is not in motion or transit, nor in use, it is described as data at rest. In this state, the data rests or resides on non-volatile media such as hard drives, optical media, flash drives, and memory cards.