The overall system is organized similar to the traditional video surveillance system. But here the mobile devices function as cameras: collect, compress, and send video to the PC server, which then may record it to disk, much like a DVR.
Client Device - a device running the ReelCam Video Security Monitor app, which turn it into a remote camera, with logic to detect motion/sound and trigger recording on the server PC.
Server PC - a computer running the ReelCam Local Server software, serves as a bridge between the client and controller devices, as well as recording video streams, and sending email alerts.
Controller Device - a device running the ReelCam Remote Controller app, which allows it to configure the server PC, and client devices. It also functions as a viewer for live and recorded video streams.
Since compression is done on the client device, and decompression is performed by the controller device when viewing, the server PC doesn't need a very powerful CPU, nor GPU (it could be headless, i.e. no display). A low end PC should suffice, as long as its network and disk speed is fast enough for writing/reading multiple streams concurrently.
Obviously, the WiFi signal and router also should be able to accommodate the bandwidth required. In the above chart, the Controller Apps are shown running remotely, but they can also be run inside your WiFi network. For backup, you could use DropBox to mirror the video directory on your server PC to the cloud.
The system is very flexible, in that you can configure it in many ways. In the chart below, you can choose to have the server PC be in the cloud instead of inside your local network. Obviously, you would need to take steps to secure your server from hackers. So this is more in the realm of experts. But using this set up would allow you to easily have multiple client devices at various geographical locations, not confined to one particular site.