second string vs team

second string and team both are nouns.

Word	Noun	Adjective	Verb	Adverb
second string	Yes	No	No	No
team	Yes	No	Yes	No

As nouns, team is a hypernym of second string; that is, team is a word with a broader meaning than second string:

second string: a squad of players that are available either individually or as a team to relieve or replace the players who started the game
team: a cooperative unit (especially in sports)

Other hypernyms of second string include squad.

second string (noun)	team (noun)
a squad of players that are available either individually or as a team to relieve or replace the players who started the game	a cooperative unit (especially in sports)
	two or more draft animals that work together to pull something

second string (verb)	team (verb)
	form a team