There are multiple USB connection modes available, depending on the device and the Android version used:
- Charge only, as the term suggests, uses the power provided from the host system to charge the device via USB. While this is done in all other modes as well, if selecting Charge Only no other action takes place.
- In USB Mass Storage mode (UMS), the device is presented to the computer like any other USB hard drive. The computer obtains "block level" access -- as if it really was a locally attached hard drive. As a result, when connected in USM mode, while the storage presented to the computer (usually the sdcard) it is not available on the Android device itself.
- HTC Sync (available on HTC devices only, obviously) is a mode used by HTC software on the computer to synchronize data between the Android device and the desktop
- Camera (PTP) is a mode available e.g. on the Google Nexus 7. In this mode the Android device behaves as if it were a camera, so you can transfer images to your computer.
- MTP (Media Transfer Protocol) is an extension of the aforementioned PTP. Here the entire storage is made available to the computer, so files of all types can be transferred. However, connection is based on logical rather than physical access, as opposed to the UMS mode mentioned above. MTP usually requires special drivers to be installed (at least on Linux/Mac computers).
Not all described modi are available on all devices, and there might be other (additional) modi not mentioned here.