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In D&D, some creatures and player races have dark vision. Can a character or monster with dark vision see in pitch blackness?
In real life animals with dark vision see by reflecting the light off their eyes, so if it is too dark, like in a cave with no exits for example, they can't see. Of course D&D is a game about magic and fantasy so the real word doesn't necessarily apply here.
The example you use of real-life animals seeing in the dark (such as cats) have low-light vision, which extends their vision where there is some poor lighting available to see with. If you took a real cat and put him in a completely sealed box (a flask of poison is optional), the cat would be unable to see anything.
In D&D, cats have the low-light vision ability, not darkvision, so their vision mimics reality and the example you provided. Darkvision explicitly states that the creature/character can see in the dark when no light is available. It should be noted, however, that darkvision can only see shades of black and white.
Yes, darkvision explicitly states that creatures with it can see in pitch blackness, although it is worth noting that a magical Darkness renders even darkvision useless. Ellesdil's answer has more information on this.
The reason why darkvision works like this is because it is a redone version of infravision, from earlier editions of D&D, that let characters see in the dark by seeing heat spectra. Obviously, this works even if there is no light at all. However, since the way it was implemented varied from table to table - some groups played it as the modern darkvision, while others looked more at how infravision really works - it was standardised and renamed for 3rd edition and onwards, becoming darkvision.
