I love barbershop music ! Ocassionally its sung by fewer than four voice parts.
My dad is a barbershop singer but I barely knew him and so I don't know much about that at all. How old is that type of music?
The music itself began during the 1930s and is still present today !
My dad is nuts and I always thought it was weird that he sang that kind of music. Is it very popular?
Yes, particularly female barbershop quartets are popping up.  They are called the "Sweet Adelines" quartet 
Weird ok. I wonder where I can buy such music
I am not sure, although one characteristic feature of the barbershop harmonyis the use of what is knows as 'snakes ' or swipes'  Its a harmonic chord.
Do they always have to dress up in those silly little outfits?
a group of four singers with one on each vocal part, or a barbershop chorus, which closely resembles a choir with the notable exception of the genre of music.
According to the Barbershop Harmony Society (BHS), "Barbershop music features songs with understandable lyrics and easily singable melodies, whose tones clearly define a tonal center and imply major and minor chords and barbershop (dominant and secondary dominant) seventh chords that resolve primarily around the circle of fifths, while making frequent use of other resolutions." Slower barbershop songs
to avoid awkward voice leading, in tags or codas, or when some appropriate embellishing effect can be created. Occasional brief passages may be sung by fewer than four voice parts.
Barbershop music features songs with understandable lyrics and easily singable melodies, whose tones clearly define a tonal center and imply major and minor chords and barbershop (dominant and secondary dominant) seventh chords that resolve primarily around the circle of fifths, while making frequent use of other resolutions. What sets barbershop apart from other musical styles is the predominant use of