The pair of words "reward" and "rewarding" can be very tricky. Especially when it's not clear for you which one is the verb and which one is the noun - or even an adjective? Let's help to get your thoughts in order, shall we?
Reward vs. Rewarding
Regarding their meaning, in lexical terms, "reward" and "rewarding" are part of the same lexical family. This explains why their meanings are very close to one another. Both words refer to some kind of prize, satisfaction or appreciation - whether it's material or intangible - gained after doing some good or hard work. Regarding their use in sentences and phrases however, is where the similarity ends.
When do we use "reward"?
"Reward" can be used either as a verb or as a noun. As verb, the word refers to the action of offering a reward, a prize, to someone. As noun, it can either define a good thing (object/diploma/money/verbal appreciation etc.), a good experience, achieved after hard, excellent work, or as money given by police to someone who provides information about a crime.
Example 3: There will be a reward for whoever provides useful information about the crime. - as a noun, "reward" can also be used to define the money given by police to one who provides information regarding a crime.
When do we use "rewarding"?
"Rewarding" is much simpler to understand and remember than "reward", as the former doesn't carry a range of grammatical roles as the latter. "Rewarding" can only be used as an adjective. And it describes the feeling of satisfaction gained by one after doing something very well. Here's an example to get a better idea of how to use "rewarding" correctly in a sentence:
Example: Helping other people is one of the most rewarding feelings one can experience. - "rewarding" can only be used as an adjective and it means feeling satisfaction for doing good or worthy things.
Here's a trick so you easily remember how to correctly distinguish "reward" from "rewarding": You can often see words ending in "-ing" turn from nouns into adjectives. Just like it happens for "interesting" or "challenging". Just remember that "rewarding" does the same: it is always the adjective formed from the noun/verb "reward", plus the "-ing" ending.