As the name suggests, this a group of masculine nouns which predominantly denote living beings. Although they are occasionally confused by students with adjectival nouns, they decline in a different way: weak masculine nouns add "-(e)n" throughout the plural and in all singular cases other than the nominative.
Examples
Ihr Bruder ist Student.
(Her brother is a student (= nominative).)
Das ist ziemlich viel für einen Studenten.
(That's quite a lot for a student (= accusative).)
Meine Schwester ist mit einem Studenten verlobt.
(My sister is engaged to a student (= dative).)
Alle Studenten müssen nach dem Essen abwaschen!
(All of the male students (= nominative) must wash up after the meal!)
Wie ist Ihr Name?
(What is your name (= nominative)?)
Auf welchen Namen geht das Zimmer?
(In what name (= accusative) should I book the room?)
