Don't use Cryptoloop!
Cryptoloop is an old mechanism for disk encryption in the Linux kernel. Disk encryption technology is typically used to prevent unauthorized access to data at rest, e.g., data stored on physical media such as hard disks. Disk encryption software, such as Cryptoloop, would encrypt all data written to disk.
Cryptoloop acts as a loop device driver that makes an underlying file or filesystem appear as a pseudo-device. The Cryptoloop driver encrypts data written to the pseudo-device on-the-fly before it is written to the underlying file or filesystem. It can thus be used to add encryption to any of the standard Linux filesystems without changing the filesystem code itself.
However, Cryptoloop was deprecated in Linux 2.6, is no longer maintained actively, and is vulnerable to watermarking attacks, a type of known-plaintext attack which make it possible to detect the presence of specially crafted files with a watermark bit pattern on encrypted file systems, even without access to the decryption keys.
The method of choice for disk encryption in modern Linux versions is dm-crypt and LUKS.
The Ubuntu installer can encrypt new ubuntu installations using dm-crypt and LUKS if the option is selected in the install wizard. Information on setting up full-disk encryption in Ubuntu manually can be found on the Ubuntu wiki.