Difference between CSV and XLS files?

CSV : means Comma Separated Values. it is a plain text (ansi) format.
XLS : is the main binary file format for all eXceL SpreadsheetS

CSV files hold plain text as a series of values (cells) separated by commas (,) in a series of lines (rows). You can actually open a CSV file in a text editor and read it yourself. Many applications are capable of reading CSV files, and many languages provide built-in functions that simplify reading/writing CSV format.

XLS is an MS Excel workbook binary file, which holds information about all the worksheets in a workbook, comprising both content and formatting (number masking, colouring, conditional formatting, etc), and can also hold additionals like charts, images, etc. XLS files can only be read by applications that have been especially written to read their format, and can only be written in the same way.

CSV files are intentionally designed to be widely supported; any OS or application that imports or exports data usually has CSV support.

They do nothing else but hold data - no text formatting for example.

Excel files hold the same data, but in binary format. This allows the file to save specifc Excel features - charts, formatting, etc.