It lets you interactively browse packet data from a live network or from a previously saved capture file.
See: http://www.ethereal.com for new versions, documentation, ...
Ethereal's native capture file format is libpcap format, which is also the format used by tcpdump and various other tools. So Ethereal can read capture files from:
There is no need to tell Ethereal what type of file you are reading; it will determine the file type by itself. Ethereal is also capable of reading any of these file formats if they are compressed using gzip. Ethereal recognizes this directly from the file; the '.gz' extension is not required for this purpose.