If you understand the principles behind capturing, you can "unpack" them to discover their consequences-- stones that have no liberties are removed from the board, therefore you can't play on a point without liberties, unless you gain liberties by capturing. At the bottom of the suitcase are the logical conclusions of these principles: life and death.

The black group of stones in the upper left corner (below) has two separate points of territory. Even if their outside liberties are blocked completely, White cannot capture these stones. For White to capture, the black stones first must be in atari. But these stones can never be placed in atari.

Even after Black is surrounded completely, White cannot play at 1, because a stone there is just taken off. Since you can't play on a point without liberties, or gain any liberties by capturing, these black stones cannot be put in atari. Any group with at least two separate points of territory cannot be put in atari, and so cannot be captured. Stones which cannot be captured are said to be alive.

Even if a group has two points of territory, if the two points are not separated, the group is not alive.

The black stones in the upper left corner have two points of territory, but the points are not separated. These stones can be captured.

To take out these stones, White can play 1. This is possible because White 1 has a liberty.

The black stones are now in atari, so they can be captured. Black can capture with 2, but Black is still in atari. White can capture all the stones with 3. Stones that can be captured are dead.

Stones must have at least two separate points of territory for life. These two separate points are called eyes.