Any input would help.
Matt
The quick and dirty answer is that songs have words and lyrics while tunes do not. Songs imply the presence of a singer; tunes are simply instrumentals. that’s why some folks cringe when others mix up the terminology.
courtesy of www.m-w.com
Main Entry: tune
Pronunciation: 'tün, 'tyün
Function: noun
2 a : a succession of pleasing musical tones : MELODY b : a dominant theme
Main Entry: song
Pronunciation: 'so[ng]
Function: noun
3 a : a short musical composition of words and music
5 a : a melody for a lyric poem or ballad
Most of the definitions for tune imply a melody line…most of the definitions for song imply either a lyrical poem, something sung, or music intended to go along with singing.
…so you’re sayin’ that it ain’t “chune”? 
So what about those things someone plays that may have (or had) lyrics, but that are mostly played on instruments now? I’ve been learning tunes from Ireland’s Best Tin Whistle Tunes and The Complete Irish Tinwhistle Tunebook and so on only to find later that many have lyrics somewhere or other. Do I call them tunes or songs? Are the titles of the books, technically, wrong? Help.
Then you’d be playing the tunes of those songs. 
Got it?
My New Oxford Dictionary says:
Song - short poem or other set of words set to music or meant to be sung / musical composition suggestive of a song.
It gives the archaic meaning of song as ‘poetry’. In the Bible there is a “Song of Solomon” that is a poem.
Tune - a melody. Origin late Middle English unexplaned alteration of ‘tone’.
Just to expand this answer a bit, when you sing a song you sing it to a melody. That melody is a tune.
Sometimes vocal music doesn’t involve songs. Mouth music, scat singing or diddling involves singing tunes which are not songs.
Except doesn’t mouth music usually have real words? In which case it’s more of a song that functions as a tune.
To muddle matters more, in Newfoundland there is a strong tradition (which I think comes from Ireland) of singing words to tunes. You’ll play the tune a few times through, and then take one part and sing it instead. The best known example is probably “Auntie Mary” (aka “Cock of the North”): “Auntie Mary had a canary up the leg of her drawers…” But there are lots more: “Coming from the Races”, “Said She Wouldn’t Dance”, “I’ve Got a Bonnet Trimmed With Blue”, “Mother Wouldn’t Beat 'Em”, “Uncle Manual Milks the Cow”, etc.
Good point. Yes it does. Often the words make little or no sense, but when that happens in rock and roll we still call the results songs.
That Newfoundland tradition you mention has a parallel in jazz called vocalese; ie, people putting words to jazz melodies, even solos. Jon Hendricks was the master of the art of putting words to tunes and solos. Now, would you call what results a song? I don’t think so, but I’m not sure. And part of Hendricks’ art consisted in his words making a lot of sense and often being very funny.
It gets curiouser and curiouser.
In a lot of indic music not only can one tune be shared by many many lyrics (as is the case in European music) but also the one lyric may be sung to different tunes. Thus, in many cases (especially in classical or “art music”), it is difficult to classify a melody by the name of a lyric because there is great flexibility applying different tunes to any particular lyric. Lyrics tend to be classified by their innate meter and tunes by their innate modes.
One of the main words for “music” in the indic tradition is “sangeet” which literally means “assembly of song”. There is a lot of lyric content in the songs but, then again, a lot of the classical music involves vocalese of no semantic content. Still they are “geet” or song because they are sung. All instrumental music in the indic tradition is premised on vocal music and hence even flute muisc is considered as flute song etc.
Ha! I’m Jon Hendricks’ webmaster. I would certainly call his songs songs.
On the Newfoundland angle, I’m sure which came first, the tune or the song – actually, the answer may be different in different cases. I know “Uncle Manual” was tune first, but that’s the only one that I know of which has a clearly identified “lyricist” (Rufus Guinchard). On the other hand, the tune “Said She Wouldn’t Dance”, borrows its lyrics from the song “The Town of Ballybay” – but I’ve no idea what the historical relationship between the two is.
Amazing. I never know whether to think ‘small world’ or ‘oh well, everybody hangs out here after all.’
Yep, no point in not calling them songs. The waters are muddy enough without needlessly worsening matters.
BTW, what a talent Jon Hendricks has.
And what yet of “Chune”?
I had a lovely experience in Innishere playing a “chune” requested by the accordian busker at Dun Angus.
Ah, but baggins_21’s question was “Why”, for which we go to the
Etymology (also from m-w.com):
Song
Etymology: Middle English, from Old English sang;
akin to Old English singan to sing
Tune
Etymology: Middle English, alteration of tone
So, it looks like from the beginning, the word “tune” was based on the
tones (the notes) themselves, while the word “song” came from the
act of using the voice.
Interestingly the etymology of tone is “Middle English, from Latin
tonus tension, tone, from Greek tonos, literally, act of stretching;
akin to Greek teinein to stretch” …makes me think of string instruments
(which you tune by stretching a string).
Absolutely. One of the greatest lyricists, a fine, fine singer, and a very nice man as well.
Though he usually forgets to tell me about gigs so I can put them on the webpage. 
String is chorda in Latin from the Greek … makes me think of vocal chord and therefore voice
Touche’.
tune = melody
song = words
…so you take requests, eh? 
…just shutup and play…
-Anon-