`.bak` generally designates that the file is a backup copy of something, but other than that it gives preciously little information as to the actual file type.
Try looking at the output of the `file` command, which studies the first few bits of the file to see if it recognizes it as a known filetype:
caleburn: ~/ >file image001.jpg
image001.jpg: JPEG image data, JFIF standard 1.01
caleburn: ~/ >file oops.png
oops.png: PNG image data, 935 x 546, 16-bit/color RGB, non-interlaced
caleburn: ~/ >file zones.zip
zones.zip: Zip archive data, at least v2.0 to extract
caleburn: ~/ >file eth2.pcap
eth2.pcap: tcpdump capture file (little-endian) - version 2.4 (Ethernet, capture length 96)
And so on, and so on. Once you know what type of file linux thinks it is, google should be able to suggest how to access it.
... Alternately, you can ask whoever sent it to you what the original filename was supposed to be and find out that way. :)