shell(2)

 Unix-like systems such as linux always have a single file system tree, regardless of 

how many drives or storage devices are attached to the computer.Storage devices 

are attached( mounted ) at various points on the tree according to  the system administrator.

 

absolute pathnames

an absolute pathname begins with the root directory and follows the tree branch by branch 

until the path to the desired directory or file is completed.

 

relative pathnames

a relative pathname starts from the working directory.

The "." symbol refers to the working directory and "the" sysmbol

refers to the working directory's parent directory.

actually, you can omit the "./". 

 

Linux has no concept of a "file extension" like some other operating systems.