Even though most ammunition "names" such as .22, .25 and .38 refer to the diameter of the round expressed in decimal inches, the .38 Special and .357 Magnum rounds are the same diameter. The differences are in length, powder loading and bullet size and weight. In those aspects, the .357 Magnum is bigger, heavier and more powerful.
All revolvers manufactured to use .357 Magnum ammunition can be safely loaded and fired with .38 Special ammunition. The reverse, obviously, is not true. You cannot use .357 Magnum ammunition in a .38 Special weapon.
There are some details to consider when firing .38 Special ammunition from a .357 handgun. First, because the shell casing of a .38 Special is shorter than that of a .357 Magnum, the cylinder bores will gradually acquire a buildup of residue sufficient to impede the proper seating of a .357 Magnum round. In other words, after you shoot enough .38 Specials out of your .357, you'll arrive at a moment when it is difficult or impossible to "force" a .357 Magnum round into the cylinder bore. And, since you never should force anything in a firearm, this indicates a need for a good cleaning. Gun stores sell special spirally-wound steel brushes that do a good job of cleaning the cylinder bores when used in combination with a commercial gun cleaning solution.
Because the .38 Special is shorter than the .357 Magnum, semi-automatic feeding mechanisms designed for .357s may not be capable of graceful loading of .38 Specials. It is not a good idea to load .38 Specials into a .357 Magnum lever-action rifle.
Some .357 Magnum handgun owners like to fire .38s at a target range because they are cheaper, a little less noisy and produce less recoil when fired from a heavy-frame .357. All of those statements are true, and it is unquestionably safe to use .38 Special ammunition. But if a person habitually target-shoots .38s and then loads conventional .357 Magnum ammunition for "carrying," some experts say that the first shot fired in earnest will be quite an unaccustomed surprise to the shooter, and could lead to enough of a startle reaction to impede follow-on shooting or other defensive behavior. The same objection is sometimes raised against reloaded wad-cutter .357 ammunition. Those who adhere to this belief advise always using factory-grade .357 Magnum ammunition in a .357 Magnum firearm, even for target shooting.
To sum this up, .38 Special ammunition can always be used in any revolver manufactured to shoot .357 Magnum ammunition, but the reverse is not true. You cannot load .357s into a .38. And it is asking for trouble to load .38 Specials into any semiautomatic or autoloading weapon designed for .357 Magnum rounds.
New shooters may be surprised to find wide differences in ammunition prices. Some large discount sporting goods stores sell factory-grade .357 Magnum ammunition for the same price, or even lower, than some shooting ranges charge for wad-cutter reloads. Shop around and be sure to use "safe" ammunition in your weapon.