hi..
About voip you can get useful information here
electronics.howstuffworks.com/ip-telephony.htm
Media Gateway : The MGW handles the media, or bearer, interface. It converts media from the format used in one network to the format required in another network. For example, it can terminate the TDM trunks from the PSTN, packetize and optionally compress the audio signals, and then deliver the packets to the IP network using the Real Time Protocol (RTP)
Softswitch: is a general purpose computer running specialized software that turns it into a smart phone switch. Softswitches cost significantly less that tradition circuit switches and can provide more functionality in particular, in addition to handling the traditional circuit switching functions, Softswitch can convert a stream of digitized voice bits into packets, this opens up a number of options for transmission, including voip approach.
In any telephone network, the most complex element is the software that controls call processing. This software performs call routing and implements call-processing logic for hundreds of custom calling features. This software runs on a proprietary processor that integrated with the physical circuit-switching hardware, a more flexible approach is to physically separate the call processing function from the hardware switching function. In Softswitch terminology, the physical switching function is performed by media gateway and the call processing logic resides is a media gateway controller.
Note: my answer is depended on the documents that I have in my job in a 3G operator when we use Softswitch terminology. I have no idea about using Softswitch in Data Networks.