Bicycles/Maintenance and Repair/Tools and Supplies/Pedal Wrench
A standard "pedal wrench" is an open-ended wrench designed to be thin enough to fit the narrow wrench flats (gripping surface) typical to pedals. A quality pedal wrench will be long enough (and preferably with angled openings) to provide significant leverage, and durable enough to allow repeated application of such force. Pedal wrench flats are typically 15mm in size. 9/16" (~14.3mm) is somewhat common on older pedals. 17mm and other sizes have been used, but you aren't very likely to encounter them.
(A "cone wrench" is thinner and shorter than a pedal wrench, and unable to provide appropriate durability or leverage for use on pedals. A standard open-end wrench (such as found on one end of a combination wrench) will sometimes fit on the flats, but is unlikely to provide proper leverage, while being likely to spread at the tips, ruining the wrench and possibly the flats.)
Pedal wrenches are also available for pedals with 6mm or 8mm internal hex ("Allen wrench") fittings in the end of the spindle, accessed from the back of the crank arm, through the pedal hole. A significant portion of modern pedals provide only this fitting, with no traditional wrench flats. These pedals wrenches differ from standard "Allen wrenches" primarily in that they provide longer and more comfortable handles, which is important to allow application of needed leverage.
In the U.S., there are two diameters (sizes) of pedal spindle (axle) threads which you are likely to encounter: 1/2" and 9/16". The first is generally used on bikes with a one piece crank, which will normally be an inexpensive or older bike. The second will usually be found on bicycles with two or three piece cranks, which includes many current multi-speed bicycles and higher quality older bikes. There are other pedal thread sizes, such as 14mm (French), but you are quite unlikely to encounter them in the U.S.
Left-hand pedals are reverse threaded. (That is, you turn clockwise to loosen, counterclockwise to tighten. The Wright brothers introduced this to prevent pedals from unscrewing on their own.) Make very sure it's not cross-threaded, you are turning the correct direction, using the correct pedal, and use grease! (If any of the threads are stainless steel or titanium, it would likely be better to use an appropriate "anti-seize" compound instead of grease.) Pedals should be properly tight, 26 ft-lbs being a generic (not perfect) torque specification. (Rotational force equivalent to 26 pounds on the end of a foot long lever (the wrench), 52 pounds on a 6" lever, etc.)