The START command:

Windows NT 4/Windows 2000 Syntax

Starts a separate window to run a specified program or command.

"title"

Title to display in window title bar

/Dpath

Starting directory

/I

The new environment will be the original environment passed to the cmd.exe and not the current environment.

/MIN

Start window minimized

/MAX

Start window maximized

/SEPARATE

Start 16-bit Windows program in separate memory space

/SHARED

Start 16-bit Windows program in shared memory space

/LOW

Start application in the IDLE priority class

/NORMAL

Start application in the NORMAL priority class

/HIGH

Start application in the HIGH priority class

/REALTIME

Start application in the REALTIME priority class

/WAIT

Start application and wait for it to terminate

/B

Start application without creating a new window. The application has ^C handling ignored. Unless the application enables ^C processing, ^Break is the only way to interrupt the application

command / program

If it is an internal cmd command or a batch file then the command processor is run with the /K switch to cmd.exe. This means that the window will remain after the command has been run.
If it is not an internal cmd command or batch file then it is a program and will run as either a windowed application or a console application.

parameters

These are the parameters passed to the command/program

If Command Extensions are enabled, external command invocation through the command line or the START command changes as follows:

Non-executable files may be invoked through their file association just by typing the name of the file as a command. (e.g. WORD.DOC would launch the application associated with the .DOC file extension).
See the ASSOC and FTYPE commands for how to create these associations from within a command script.

When executing an application that is a 32-bit GUI application, CMD.EXE does not wait for the application to terminate before returning to the command prompt. This new behavior does NOT occur if executing within a command script.

When executing a command line whose first token is CMD without an extension or path qualifier, then replaces CMD with the value of the COMSPEC variable, thus avoiding picking up random versions of CMD.EXE when you least expect them.

When executing a command line whose first token does NOT contain an extension, then CMD.EXE uses the value of the PATHEXT environment variable to determine which extensions to look for and in what order. The default value for the PATHEXT variable is:

.COM;.EXE;.BAT;.CMD

Notice the syntax is the same as the PATH variable, with semicolons separating the different elements.

When executing a command, if there is no match on any extension, then looks to see if the name, without any extension, matches a directory name and if it does, the START command launches the Explorer on that path. If done from the command line, it is the equivalent to doing a CD /D to that path.