An EDC bag is something that you can carry with you on a day to day basis. It can be as small or as large as you want it to be. It is something that is separate from your bug out bag, and your car emergency kit. It is something you can carry to work, to a friend's house, or on a trip to a neighboring town. What you carry in your EDC bag should be tailored to you specifically. I will show you what I keep in my bag in an effort to assist you with setting your bag up.
The purpose of my EDC bag is to provide me with the everyday items I would need in any given (non-survival) situation. Some might call them man-purses, and those people would be pretty much right. You can use any bag for your EDC bag. From over the shoulder messenger bags, to Maxpedition bags, to backpacks, or even fanny packs. I chose to go with my OGIO backpack because it is easy to carry, small enough to pack around, large enough for my items, and it doesn't stand out.
Note: Not pictured is my laptop & charger that would also go in the bag. That particular backpack has a padded laptop pouch for it.
In the above photograph I have a few everyday items. Starting with an extra pair of glasses, which is extremely important for anyone who wears them. I have a notepad, pencils and pens, a USB drive, extra batteries, lighter, ear plugs, electrical tape, chapstick, extra magazine, a bandanna, Tide-To-Go pen, small flashlight and some lotion. The ear plugs have came in handy for me, along with the USB drive.
On any given day I might be out with multiple electronic devices. My Blackberry, bluetooth headset, or iPod. I keep a spare car charger and wall charger for all devices. If you take off somewhere and forget your chargers at home, don't worry you've got a spare in your EDC (which you hopefully brought). I purchased a generic car charger and wall charger with USB charging ports. This way you can charge any device as long as you have the USB cable with it.
Next is fairly obvious, my personal protection and a microfiber cloth to clean up a spill, or anything you might need a soft rag for.
Here I have an emergency poncho, garbage bag, zip-lock bags, small radio, wallet, gum, safety glasses, a good flashlight and my multitool. The garbage bag is to protect the backpack in case of rain, and the zip-lock bags are for my Blackberry or other devices to ensure they don't get wet. I like to keep the small radio also if I need to check the latest news or weather in the absence of cellular signal.
Finally I keep my pair of sunglasses in a secure hard case, empty water bottle, first aid kit, deodorant, chewing gum, and some food bars. The first aid kit also includes a spare pair of contact lenses with solution and the case. The food bars are in case I'm out somewhere and don't want to or can't purchase food. I can have something to fill my belly with.
Hopefully this has helped you get an idea about what an EDC bag is all about. For me it carries things I would need on a day to day basis, without all the survival gear. I save my survival gear for my bug out bag, and my EDC bag contains my preparedness gear that I would need on any given day. Most of the items are things you would have lying around and if not they can all be purchased at your local supermarket for cheap.