Fifth edition D&D lacks clear, explicit rules about these subjects.
I wrote a couple of posts about them already: here and here.
But these are long. Here is the TL;DR version:
- Torches are conspicuous (like lanterns, candles, etc.). They can be seem from a mile away in the dark. Even in dim light, they'll probably be quickly noticed unless there are brighter lights around, Which means, basically, that you cannot sneak around while carrying your own sources of light, and you can NEVER surprise someone in these circumstances - unless you opposition is asleep, blind (blindsight), etc.
- Large groups are noisy. When making stealth checks, do not make it a "group ability check". Each PC rolls separately. If the thief succeeds and the paladin fails, the opposition only sees the paladin at first. Which might be a good opportunity for the thief.
- If no one is sneaking or carrying torches, encounter distance is defined by darkvision (usually 60 feet). If both parties have darkvision 60 feet, the encounter begins at a distance of 60 feet. If you randomly encounter a monster with better darkvision than you, it sees you first. It decides whether to approach or not... or to attack from a distance. With a good perception check, you might hear a noise, take cover, etc.
- Special circumstances may change this - if there is a door or other obstacle, for example, both parties could hear one another though the door.
- Inside narrow dungeons, encounters happen in corners. If one side is carrying a torch, you might still see the light before turning the corner, but it would make things less obvious.
- Unless you avoid this on purpose, if someone in your group has a torch, the whole group will be spotted easily in the dark.
- Intelligent monsters that have darkvision may use torches anyway, unless they are expecting an attack by enemies that do not have darkvision. This is a bit more complicated. Darkvision does not ignore the darkness, it makes it milder (like "dim light" - which causes disadvantage in perception checks). In addition, you cannot see colors in darkness, only shades of gray.
It seems creatures with darkvision would prefer live in dim light if possible - they keep their edge against diurnal creatures and can see well enough. However, most sources of light in the game create bright light in a small radius and dim light in a bigger radius - potentially forcing goblins, for example, to live partially in bright light.
- Monsters with keen senses may have different rules. If you use the rules as strictly written, a goblin should fight a wolf in the dark if given a choice. However, wolves have keen hearing and smell, and goblins have bows. A goblin village threatened by wolves might prefer to surround itself with torches if the night is dark, to see the wolves approaching (without disadvantage) and take them down from a distance.
If the goblin village is threatened by giant bats (with blindsight), they would DEFINITELY use torches.
However, a goblin village surrounded by aggressive HUMANS would never use torches, for similar reasons - humans rely on sight MORE than goblins, and would have a hard time approaching without being noticed.
Most of this is common sense, but easy to forget when you're playing the game and looking for actual rules.
In addition, all this stuff might hinder your fun, specially if the entire group suffers because one single PC doesn't have darkvision. If that is the case, decide what is best for you. Many groups hand-wave the whole notion of light and darkness... but using it right can lead to awesome situations.
If you want to support this blog, check out my books! Most are compatible with 5e.