Mann vs. man
In German, Mann and man look and sound very similar, but they are completely different words with distinct meanings and grammatical functions. Confusing them is a common mistake for learners, but the difference is straightforward: one is a noun and the other is a pronoun.
Mann
A1This is a noun that is always capitalized. It primarily means man, referring to an adult male person. It can also mean husband. As a noun, it has a plural form, Männer, and can be used with articles and adjectives.
Der Mann wartet auf den Bus.
(The man is waiting for the bus.)
Mein Mann kommt heute später nach Hause.
(My husband is coming home later today.)
Ich habe gestern einen großen Mann gesehen.
(I saw a tall man yesterday.)
Viele Männer interessieren sich für Autos.
(Many men are interested in cars.)
Der Job ist nichts für schwache Männer.
(The job is not for weak men.)
Ein echter Ehrenmann hält sein Wort.
(A true man of honor keeps his word.)
MAN
A2This is an indefinite pronoun that is always written in lowercase (unless it starts a sentence). It is used to make general statements about people in general. It can be translated into English as one, you, they, or often by using the passive voice. It is always followed by a verb in the third-person singular.
In dieser Bibliothek muss man leise sein.
(In this library, you have to be quiet.)
Man sagt, dass Bewegung gesund ist.
(They say that exercise is healthy.)
Wie spricht man dieses Wort aus?
(How does one pronounce this word?)
Hier darf man nicht parken.
(Parking is not allowed here.)
Man kann nie wissen, was die Zukunft bringt.
(One can never know what the future holds.)
Man sollte mehr Wasser trinken.
(One should drink more water.)
Summary
In short, the key difference lies in their function and meaning. Use the capitalized noun Mann when referring to a specific man or husband. Use the lowercase pronoun man to make a general statement about what one, you, or people do. Mann is a specific person; man is a general, unspecified person or group.