I have a confusion with the perception skill:
We are in a dungeon, my rogue wood elf character is walking straight while looking for everything suspicious, like traps or anything. Should I roll for perception, or for passive perception?
I am asking this because my DM keeps telling me that it is for passive perception. However, I think its for perception because I am doing the action, and not just walking by distracted.
First: you should roll only when your DM asks for a roll. See p.174 of the PHB: "the DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure." There are lots of great reasons for a DM to respond to "I'm actively searching for traps as I walk" with an "okay, thanks for letting me know."
Second: you are right: if you state that you're looking around, that'd generally be an (active) Perception check. However, there are lots of factors that might change this. Your DM might think that a Knowledge or even Stealth check is more appropriate to detecting whatever's around the corner. The DM may be struggling with how to give you a fair chance at detecting things without giving away information by calling for rolls. The DM may think that even when looking around, your character doesn't have the experience/knowledge to apply the Perception skill.
Also, there should be plenty of times that you might think a Perception check could be in order, but the DM has already decided success or failure. I believe that a good part of DMing 5e is knowing when to call for a check vs. when to simply declare a result. See also 5 Simple Rules for Dating my Teenaged Skill System and Ask Angry: Passive Skills. (Be warned: as with all of AngryGM's materials, there's a bit of foul language and rudeness to sift through. Butthe wheat's good, if the chaff's... occasionally irritating.)
Zeroth: ask your DM, away from the table, how to think about this. Rulings you're not sure about should be the start of a conversation, not the end. The DM has some reason for calling ornot calling for checks. Even if it's just that he forgets about issuing those opportunities, hopefully that's a conversation you-all can have.
Passive Perception (10 + Wisdom (Perception)) is what you see all the time, it's the things your character notices, without doing anything at all. If you are just walking, most of the time this is all you are using.
If you are actively looking for things then you roll your Wisdom (Perception) roll an this is an active perception check.
Generally, in exploring, you notice things with your passive check, you don't have to roll, you just see them. If for some reason you are searching for something and didn't see it, then you should roll perception, if you roll higher than a 10 you have a chance of seeing something that you didn't spot with your passive score, if you don't roll higher than a 10, you probably aren't seeing anything new (with caveats that DMs may choose to have different DCs for passive and active checks).
Your passive perception is what you notice without searching, and you can indeed search for something, but searching takes a long time. If you were to search every square of something for anything hidden, your character could spend an hour in a single hallway.
Under "Noticing Threats" (PHB p.182) :
Use the passive Wisdom (Perception) scores of the characters to determine whether anyone in the group notices a hidden threat. [...] While traveling at a fast pace, characters take a -5 penalty to their passive Wisdom (Perception) scores to notice hidden threats.
When exploring the dungeon, passive perception is what would normally be used to detect threats. In this case, passive perception represents your character's habit of always looking over his shoulders, of checking every nook and cranny and every tunnel he encounters.
If you state that you are taking extra precautions, an active roll is definitely possible. When I DM, if the PCs tell me they wish to proceed cautiously, I use what I have dubbed the "reverse sneak rule". When sneaking, you are normally limited to a slow pace, because you are being very careful not to be seen or heard. It seems logical to me that applying the same level of care to spot hidden traps and sneaking enemies would require you to move at such a pace as well, as you are then looking behind every tree and in every shadow, stopping frequently to hear for approaching enemies, etc.
Of course, your DM can feel differently about it and always ask you to roll perception checks, which is also fine!
