To avoid confusion and to make it easier, you can put all directories in one partition. There are reasons of efficiency/security in operation for various partitioning strategies but none are essential for a newbie.
The .iso files are the data files containing the disc image. The .iso is short for ISO 9960, an international standard format for CDs.
The .md5 file is an MD5 hash code for the .iso. It is essentially a calculation based on the whole file which is almost unique. If the file you download has the same md5sum when you check it, the chances are very good that it is correct/identical to what the source intended. The .asc is a signature file from the gnupg or pgp programmes that can verify that the file was sent by the signer. GNU/Linux distros usually contain md5sum and gnupg/kgpg/or some variant. You can also use these for verifying files you send to people over the web. They add security. In the old days when the Internet was not very reliable, it was not unusual to have to try several transmissions before a file could be received intact. Nowadays, it is more usually used to verify that the file has not been tampered or malware added. How times change.
Many installation CDs have a start menu which includes file-by-file verification of md5 sums which are included on the CD. This does not detect tampering but could detect file-transmission problems.