Do you mean Mount Points? Mapped Drives?  I imagine Mount Drives are the same thing as Mount Points.
A mount point is an entire hard drive, or partition, that can be found in a directory.  For instance, you know Windows has C:\ as the root disk drive, but a mount point would be a folder name, like /primary_disk or could even just be / (linux/unix).  
A mapped drive is a network share that you've "mapped" to your desktop which then essentially becomes a drive letter.  For instance, if you run a file server and have a folder for MP3's, you could go into network neighborhood and map the MP3's folder to drive letter H: for example.  Then u could just go to My Computer and double click the H: drive to read / write data.  
Mount Points are a specific feature of 2000/XP/Linux/Unix etc... and maybe NT4 (not sure) but mapped drives can be accomplished with 9x/Me as well.
HTH 
NOTE: 
Mount Points can only be acheived in NT?/2000/XP by converting your hard disk to a dynamic disk.