Common noun:
* When referring to the animals: "The seals were basking on the rocks."
* When referring to a type of wax: "The seals on the envelope were broken."
Proper noun:
* When referring to a specific group or organization: "The Seals won the championship." (This likely refers to a specific sports team, like the San Diego Seals.)
* When referring to a specific person with the last name Seals: "Mr. Seals will be speaking at the conference."
To determine whether "Seals" is a common or proper noun in a specific sentence, consider the context and what the word is referring to.