* Cake: The vowel sound in "cake" is a short "e" sound, similar to the "e" in "bed". It is a closed vowel sound, meaning your mouth is relatively closed when you make the sound.
* Whale: The vowel sound in "whale" is a long "a" sound, similar to the "a" in "day". It is an open vowel sound, meaning your mouth is relatively open when you make the sound.
You can think of the difference as the difference between a short, closed "e" sound and a long, open "a" sound.