From the Ubuntu Geek:

eCryptfs is a POSIX-compliant enterprise-class stacked cryptographic filesystem for Linux.It provides advanced key management and policy features. eCryptfs stores cryptographic metadata in the header of each file written, so that encrypted files can be copied between hosts; the file will be decryptable with the proper key, and there is no need to keep track of any additional information aside from what is already in the encrypted file itself. Think of eCryptfs as a sort of “gnupgfs”.eCryptfs is a native Linux filesystem. The kernel module component of eCryptfs is part of the Linux kernel since 2.6.19.