Nothing will be stored persistently on the drive.
Any modification to any file / folder performed during the usage of the system will be stored in the filesystem loaded into the memory, and not in the filesystem stored into the drive.
If you want proofs, here's a comparison of the MD5 sums of the Ubuntu Desktop 15.04 64 bit ISO and of the USB drive to which I wrote it and from which I've been regularily booting for weeks:
% md5sum ubuntu-15.04-desktop-amd64.iso
% ls -l ubuntu-15.04-desktop-amd64.iso
-rwxrwxrwx 1 ubuntu ubuntu 1150844928 Aug 13 17:27 ubuntu-15.04-desktop-amd64.iso
% sudo dd if=/dev/sdb iflag=fullblock bs=8192 count=$((1150844928/8192)) | md5sum -
140484+0 records in
140484+0 records out
1150844928 bytes (1.2 GB) copied, 100.541 s, 11.4 MB/s