The 'image name' is the name of the executable file used when the process was created; this name is popularly referred to as the name of the process, though it's not really. (Why 'image'? Because programmers used the word 'image' for 'memory image file' long before it was possible to put pictures on a computer without a defense-department-sized budget).
There are two exceptions to this, the System Idle Process (pid 0) and the System process (pid 4). These are wired into the kernel, and do not have corresponding image files. They don't have image names either, so various tools such as Task Manager make up names, so you might see different names in different tools.
The System Idle Process consumes all the CPU time that no-one else wants. If no other process is ready to run (on a particular CPU), the kernel will run the idle process.
The System process contains threads used by the OS kernel for 'background' functions. For example, the working set trimmer, which may reclaim physical memory from some processes, runs as a thread in the System process. The System process may also have threads used by some device drivers to do work outside the normal path of operation. (If the System process is using a huge amount of CPU for an extended time, it usually means some device driver is to blame).
Most of the columns of data are adequately explained by Task Manager's help, so I won't repeat the information here. The memory-related data, however, could probably use extra clarification.
The Mem Usage column shows the current working set size for the process, which is to say, the amount of physical memory the process is currently allocated. Three things need to be said here: firstly, this is only somewhat related to "how big the process is". The process is probably larger than that; the OS's job is to keep only those pages in physical memory that it is useful to have in physical memory. Secondly, the number can get smaller, if the OS decides to take some physical memory away from the process ('working set trimming'), which it will do if it has some other use for the memory. Lastly, you can't add up all the numbers and get the total used by all processes; some of the physical memory is shared by more than one process.
The VM Size column shows the number of Private Bytes of virtual memory owned by the process. As always, 'virtual memory' does not necessarily mean disk or RAM; it could be either, or neither.