session.gc_maxlifetime

This value (default 1440 seconds) defines how long an unused PHP session will be kept alive. For example: A user logs in, browses through your application or web site, for hours, for days. No problem. As long as the time between his clicks never exceed 1440 seconds. It's a timeout value.

PHP's session garbage collector runs with a probability defined by session.gc_probability divided by session.gc_divisor. By default this is 1/100, which means that above timeout value is checked with a probability of 1 in 100.

session.cookie_lifetime

This value (default 0, which means until the browser's next restart) defines how long (in seconds) a session cookie will live. Sounds similar to session.gc_maxlifetime, but it's a completely different approach. This value indirectly defines the "absolute" maximum lifetime of a session, whether the user is active or not. If this value is set to 60, every session ends after an hour.