Comma-Separated Values (CSV):
These 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.
Excel (represented as XL) Spreadsheet (XLS):
It 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.