Time

Night
The Day and night cycle consists of 24 hours in the game, but 24 minutes (unless the world is speed hacked or you are frame skipping) in real life. Each night has a 1/3 chance of becoming a Blood moon, with a maximum of 1 blood moon per 7 terraria days. It is recommended new players build a house before their first night to stay in during night time. More experienced players may venture out at night without fear of being overwhelmed by zombies.

Monsters
Night can be dreaded by many players who need materials such as wood, usually to finish building a house, since stronger monsters come out at night.

The most common monster to be seen at night is a Zombie, which can be extremely annoying to new players without any armour.

Demon eyes are flying enemies who follow you everywhere and spawn at night. They can be hard to kill as they are knocked back quite far when hit by weapons, so shortswords are useless against them.

The Groom is a monster which looks like a zombie wearing a Top hat, yet has much higher health and always drops a Top hat. It can be found rarely during night-time, but it usually spawns during a blood moon.

Slimes become hostile at night time but do not spawn at night.

Bosses
Some bosses such as the Eye of Cthulhu and The Destroyer can only be fought at night, and should day come before the Boss is killed, it will escape to the top of the screen and despawn, with the exception of Skeletron and Skeletron Prime, who will instead recieve an enormous boost to attack and defense, most likely killing the player in one hit.