If you asked a child to draw a key then chances are that it would be a key with a detailed handle and a long neck coming out of it with a blade on the end pointing right down. At the same time if you asked them to draw a locksmith then he chances are that it would be of a man with a large hoop containing hundreds of these kinds of keys. This key, the one that we still think of as the archetypal key and that we associate wit locksmiths, is the 'church key' and there is a reason that it is the one that is most embedded in popular culture.
We've all seen church keys a they are the old ornate keys that are used in films. Whenever a troop of heroes call a locksmith to help them on their quest, it is usually in a fantasy film and they are normally trying to get into an old castle or fortress. Thus the key design will also be old and look like something from a fantasy history film to fit in.
The reason that film directors then choose church keys to use here is that church keys are the oldest kinds of keys most of us are familiar with. As the name suggests, they were once used to lock churches and large gates (they are sometimes also called gate keys) and back in these days few people required locks no their doors as it was before the increase in crime rates.
Thus the church key was the 'only' popular kind of key and it set the gold standard for what was to follow. However the operation of the gate key is actually relatively crude and not at all what locksmiths are capable of today - as you would expect and hope, the technology used in locks has come on a lot and we now use pin tumblers and even digital locks to provide higher levels of security.
In fact the church key only really has a few potential combinations meaning that it is relatively easy to pick and also relatively easy to bypass with a similar key. This is actually why the skeleton key came into being - a single key that could open any lock as long as it was a church key (which back in the day meant most locks). Skeleton keys look much like ordinary gate keys, except they have a flat blade with no teeth and they have either a very plain or a very ornate handle. Many locksmiths as well as criminals would use the skeleton key to bypass any lock that got stuck or jammed or where the owner had lost their key.
Today the church keys are largely obsolete - no one would choose to have a lock that can be opened by anyone when there are so many other types of lock that can offer you a much higher level of security.
However church keys do still find themselves in use in certain situations. In particular the gate key is regularly used in - you guessed it - churches and gates, but is also commonly found in small drawers and cupboards. The reason for this is that a gate or a church only require an arbitrary level of security, but also that due to its simplicity, a church key can easily be used in either very large or very small locks.