First think of a running track in a stadium which athletes go around. Then think of an traditional 33 rpm flat black vinyl LP record. That also had tracks around it in just the same way, and each song on the LP record had its own track. Even though we're now in the digital age, and recordings no longer have physical tracks in the same way, we still use the pre-digital term.

"Track" is used when talking about specific songs in an album. Example: "The second track of U2's new album is my favorite so far." "Song" is used when talking about songs in general. Example: "I like the new Taylor Swift song."

This is what they looked like on an LP (33-1/3 rpm vinyl record): http://tinyurl.com/lykevh6 I thought Su.Ki was 100% right, but the earliest LP I own calls them "bands," not "tracks." By the time I was acting as an engineer at the MIT student radio station in the 1960s, we called them "tracks." I think the word "tracks" was influenced by magnetic tape technology. Tapes were recorded longitudinally. In the early days, recording head used the full 1/4" width of the tape. Over time, recording heads were modified and tapes would have two or four narrow "tracks" running side by side. And perhaps some will remember "8-tracks," popular for automobiles, which played prerecorded tapes with (of course) eight tracks on them. A record sleeve touted the virtues of records (in the days when they were competing with cassettes and 8-tracks) and says that records "give you selectivity of songs and tracks." https://cutcopyandpaste.files.wordpress.com/2013/02/it-always-happens-first-on-records.png While we are on the topic... in the 1950s, a record "album" really was an album. It might hold six or eight 78 rpm disks, 10" or 12" in diameter. Each disk was kept in an envelope made of very tough, stiff, brown paper, and the envelopes were bound or hinged so that you could turn them like the pages of the book. It really was an "album." My folks had a few albums, one of Mozart's 40th Symphony and one of Tschaikovsky's Nutcracker Suite. A single 78 rpm record cost $3 or $4 so these albums most have cost $25 or so... equivalent to about $200 today. And those shellac records were fragile. If you accidentally dropped one, there was about a 50% chance that it would break, typically leaving you with several irregular pie-slice pieces precariously held together at the center by the glued label.

The thing is, "song" means something that is sung, but that doesn't include any other form of audio. A "track" can mean music with lyrics, or a purely instrumental recording.

Su.Ki gave a full answer. I'd only add that using the "track" instead of "song" makes more sense when you're talking within the context of an album (a full musical release)