There are several commons features of both men who adopted the identity of the Atom, while distinct differences.
On Earth-One and Earth-Two, Ray Palmer and Al Pratt were entrenched in their cities' universities, located in the burgs of Ivy Town and Calvin City, respectively.
Ray was a physicist who harnessed the powers of a white dwarf star to shrink himself while controlling his weight. Al was a physicist in his later life who learned more about his superhuman abilities.
Each man was a member of their world's premier teams, Ray joining the Justice League and Al the Justice Society. Each were their group's "pint-sized powerhouse" pulling their...ahem... weight.
Both had power packed punches they delivered to their foe, Ray by manipulating his body mass while in minature state, Al by exerting his superstrength into a type of "atomic punch". In addition, on a couple of occasions Al literally shrinks to minature size such as when Brain Wave projects a beam towards the Atom and his allies and when Mallo the Cosmic Balancer shifted Ray's shrinking power to his counterpart.
There were notable differences, however. Al was a man with a short stature while Ray was normal sized. Ray was more into mental discipline while Al focused on physical training. Al charged more into battle while Ray took a more analytical approach. While Ray's palm device allowed him to convert his mass, Al's palm device shifted him to other dimensions. Ray's abilities were derived from wearing his belt made of the dwarf star material, while Al's body was saturated with tholium radiation giving him inherent powers. Both men ultimately married, although Al met his future-wife Marion "Mary" some time after Ray met his financee Jean. Additionally, Ray suffered from a mental breakdown and divorce, while Al's life was considerably calmer.
Each man was a hero, however, who earned his compatriots' respect and admiration.