Flasks are usually shaped like boxes without tops or bottoms and separated into two (or more) halves, a cope and a drag. A flask is used to hold the molding sand around the pattern as it is rammed.
There are many flavors of flasks, from the simple homemade wooden affair to fancy snap flasks (flasks that act as normal flasks for ramming, but come apart after ramming when the sand mold can stand alone and are used again for the next mold).
A mistake many first timers make is to make too large a flask. Remember, you have to roll, open, close and move the flask when it is full of sand!
After a few successful casts, you may want to cast flasks from aluminum.
Timber flasks are a great way to get casting quickly. However, wooden flasks are generally not long lived as any molten metal coming into contact with the wood WILL burn it.
The above is true - either cast some aluminum flasks, or weld some steel ones.
Finally, one of the most important points of the flask is the locating pins. The must be accurate enough to allow the cope / drag to be relocated together so that they line up exactly. Otherwise, the mold cavity will be distorted, as will the casting. The register will be off.
Having said that, the pins / mating holes must also have some tolerance. This is because sand will fall onto pins / into mating holes resulting in much trouble getting the cope / drag back together again. The normal practice is to slightly elongate the holes, and use a slight 'twisting' action as you close / open the mold. This ensures correct mating and ease of mating / opening.