Cover Vs Concealment
You need to know the difference between cover vs concealment. The base definition is rather simple; cover will stop a bullet, concealment only covers you up. However, that only tells you so much of the story.
This is important in the case of a break-in and potential fight with an armed attacker. This is also important out on the street for same, or in the case of attack by a maniac...such as usually occurs in those gun free zones that we're told are so very, very safe.
Let's get into it. We'll cover what actually CAN work as cover, and what is merely concealment, as well as what to look for in terms of both.
Cover In The Home
A good rule of thumb is that if you can easily break it, then it probably doesn't make for good cover. As a result, much of what you'll find in the home does not make for good cover.
Very few common household items can provide sufficient backstop to actually stop a bullet. As a result, you aren't likely to find effective cover except for a few of the following items:
A bookcase filled with books can serve as effective cover in the right situation. About 10 inches of paper will stop most handgun rounds, and 20 inches will stop rifle rounds. There are benefits to literacy after all!
A refrigerator MAY serve as effective cover, but the door itself is not sufficient. However, if you were to grab a refrigerator and turn it so the back - with the radiator and other components, etc - are facing the attacker. Again, MAY. Older, heavier refrigerators are also better in this regard as they have more sheet metal.
A safe makes for good cover, but it's difficult to imagine too many situations in which a safe will be useful as cover given typical placement in the garden variety home, i.e. in a closet or garage.
Any concrete walls serve as good cover as well, if any are present...but not too many homes have them. A bathtub can also provide cover if made of cast metal or hard ceramic material, but only a few bathrooms have free-standing (or large enough) tubs to provide effective cover.
Cover Outside The Home
Your first thought might be a car, but they are poor cover. Great concealment, yes, but very poor cover unless you are behind a substantial piece of the drivetrain. The rear axle doesn't take up much room, so you'll want to keep the threat between you and the engine block and transmission.
Power transformers can make for effective cover, as many are on streets in most urban areas. However, you want to keep to the edges as the core can be hollow or have gaps depending on the design.
In the office environment, you aren't likely to find any cover at all. Cubicles and doors just don't offer any, as they can't stop a bullet, or at least a bullet larger than .22 LR. If you work in an industrial setting, you may be able to find machinery or something else that's able to serve as cover, but again the same principles apply: cover can stop a bullet, mere concealment cannot.
Concealment Can Be Found Almost Anywhere
However, the typical home does offer a lot in terms of concealment. Anything that takes up the same amount of space you do can be used for concealment, whether standing or crouched.
Doors make for excellent concealment and several will be present in many homes. Doors are not sufficient cover, however, including steel-clad exterior doors. Thin steel is easily penetrated by bullets, which is why cars are not sufficient cover.
Likewise, the office and work environment does offer a lot in terms of concealment. Cubicles can hide a person, as can many other bits of furniture in the office environment. A conference table can also serve in a pinch.