So there is some confusion about skill use, particularly dialogue type skills and whatnot. We've never really gone over skills and how they're used and what they're used for, or what I consider to be a use for them. I guess we'll go over them alphabetically. Some of the following will be from the relevant skill description, and some of it will be my take on it in my campaign.
This skill is for doing acrobatic things like balancing on a wire or board across a ravine, swinging from chandeliers, serpentining out of someone's grasp or slipping your restraints, and also reducing falling damage. In my opinion, you could also use it to climb certain things that require a deft hand, rather than sheer endurance, to scale.
This skill involves magic. You use it to detect magic, identify what manner of magic you're dealing with, and also to recall bits and pieces of arcane knowledge. Sometimes I use it in my campaign as a way to say "Hey, there is magic shit going on nearby." and also to points out some Fell-Human related things. The Technomancy skill from my campaign operates in a similar manner, Eric obviously can't sense technology though because that would be silly. Heh.
This is for physical stuff that relies primarily on endurance. With Athletics you can swim, climb, and also to escape from grabs.
This is lying. That is what it boils down to. When you're trying to convince someone to do something and you're not being fully honest about it, when you're asked a question and you're attempting to deceive, if you're running a con, you use Bluff. This could have been called the Lying skill in my opinion. When using Bluff, it is opposed by the target's Insight. There's no DC, you roll and then the target rolls. You can also use it to gain combat advantage and create diversions that allow you to hide in encounters.
Diplomacy is when you're trying to convince or inspire someone with logic, honesty, grace. This is the skill for speeches and making friends, well, real friends.
Dungeoneering is the underground skill. You use it to figure out what direction you're heading in, identifying hazard, plants, animals, foraging for food, and recognizing new construction. You can also identify creatures from The Far Realm, which makes perfect sense. "I spent a lot of time in holes in the ground when I was young, so I know a lot about all the unknowable entities and crazed geometries of The Far Realm."
This skill is related to your physical robustness. You use it to resist disease, ignore hunger and thirst, and survive temperature and weather extremes. Apparently you can also use it to see if you get a sexual partner to the point of orgasm, although it was mentioned that that might be more appropriately a feat of Acrobatics.
This is for doctoring. It can be used to allow a character to use their Second Wind when it isn't their turn, to stabilize dying characters, and also to give saving throws or get a bonus on ones they make during their turn. You can also treat disease and allow a patient to use your Heal check, rather than their Endurance check, when appropriate.
You know history. This skill allows you to know things about an area's history, and to recall those details. Whenever D'alton needs to know something in Hell or about how something works in Hell, I have Jeremy make a History check to see if he recalls details from his past.
Insight, this is the big one I think people don't quite understand. First let me say that Insight is not perfect, it is your general impression of someone in regards to what it can detect. It is kind of the dialogue counterpart to Perception. Insight allows you to read between the lines of a conversation, to pick up on tone or body language, or any of a hundred other minor details about a conversation to determine whether or not you think the target is being honest, what their attitude is, what their motives might be, whether or not they are under the influence of some outside force, and so on and so forth. You cannot use Insight in the middle of a street to determine anything. You can use it before you talk to someone, while observing them, to get a sense of how you'd do in a conversation with them. When someone Bluffs someone in the group, which has happened oh, I dunno, sixty times in the past two scenarios, I use the player's passive Insight to defend against it because they never make Insight checks when they should.
When you want to scare someone into doing what you want them to, you use Intimidate. Intimidate always makes a target hostile, which can make dialogue skills like Bluff or Diplomacy useless, or cause outright violence to erupt from the target.
This skill is pretty much Dungeoneering, except with the aboveground world. You can also train natural animals with it.
This skill is encompasses your senses. You use it to listen through a door, search an area for things like tracks or secret doors, detect someone hiding, or to notice odd odors.
Irrelevant in my world. Anything you could do with Religion, can now be done with Arcana. Normally you can use it to identify cults and whatnot and also get knowledge about Immortal or Undead monsters you encounter.
This skill is for hiding and going unnoticed. There are a whole bunch of modifiers for cover, how fast you are moving, light sources, and so on. Stealth is opposed by the target's Perception roll.
Streetwise is a sort of dialogue skill. You use it to gain info and in my mind it basically amounts to wandering into a tavern and paying attention to rumors and signs and just generally kind of getting information about the area you're in. With it, you can find out what's for sale and where, who is in charge, jobs in the area, good deals in the area, and a bit of rumormongering.
This skill encompasses the skillset of any self respecting thief. With it you can disable traps, open a lock, pick a pocket, or do a magic trick.
Those are a rundown of all the skills included within the Player's Handbook. I have no idea if there are others in the myriad splatbooks produced, or if there are additional abilities for those listed above. Anyway, hopefully someone from my group will wander in here and read them and get a clearer picture of how to use skills in game. Weee. Maybe I'll print it out for next time.