Windows Explorer (also called Explorer, File Explorer) displays the entire hierarchical structure of files on your computer starting from the highest (first) level.
So, it begins to display these files from the highest level: My Computer, hard drives (C:\, D:\ ..), computers from the network, then you can go to deeper levels of each item on the tree by double clicking on it. For example, after you open Windows Explorer, you will see My Computericon, then you can double click and you will see hard drives (C:\, D:\, CD-ROM ..), then you access each hard drive letter and so on.
Double clicking on MyComputer it does the some job, but by accessing the Windows Explorermenu you will see not only My Computer items, but also computers connected through network, Homegroup, Favorites, My Documents and more items.
To access Windows Explorer, just press Windows + E keys.
Windows Explorer looks like this in Windows 8
Or like this, in Windows 7