There is a number of email class functions available in CodeIginter:
$this->email->from()
$this->email->reply_to()
$this->email->to() 
$this->email->cc() 
$this->email->bcc()
$this->email->subject()
$this->email->message() 
$this->email->set_alt_message() 
$this->email->clear() 
$this->email->send() 
$this->email->attach() 
$this->email->print_debugger()

A CodeIgniter library is simply a class file that abstracts functionality into easy-to-use functions tha take much of the strain off the developer. The term "class" and "library" are used interchangeably in CodeIgniter.

A codeIgniter session is simply an array containing the following data:

1. The user's unique Session ID
2. The use's IP address
3. The user's User Agent string
4. The "Last activity" timestamp

Session ID: This is a randomly-generated number, which is hashed with MD5 for portability, and is regenerated every five minutes.

CodeIgniter supports flashdata - data that is only available for one server request, and then removed. Flash data is very useful, and is usually used for informational messages such as "You hava been logged out" or "Removed user account". Flash variable are prefixed with flash_.