"is not running" and "stop/waiting" - what's the difference?

Why does these 2 return different output, although they're both services?

$ sudo service cassandra status
 * Cassandra is not running

$ sudo service mongod status
mongod stop/waiting


$ service --status-all
 [ - ]  cassandra
 [ + ]  mongod

And why does the 3rd output says mongod is actually running ([ + ])?