A song is a lyric set to music. A pop song is a specific type of song whose creators want it to be as popular as possible, without any pretense of art. This can be a useful goal, because you can quantify how many people buy or download a song, whereas you can't quantify artistic merit. You can argue with friends over whether a song is good or not, but you can't argue over whether it's popular or not.
Although the term pop originates from the word popular, it's widely accepted that popular music is a more general term that includes rock, hiphop, techno, house, and pretty much anything else that isn't classical or jazz. The more specific term pop music, on the other hand, is used to talk only about music designed to be as popular as possible.
Although the term pop music is widely used, the term pop song is arguably more accurate. The lyric is a vital part of the formula for successful pop songs, a fact that the top ten charts will attest to at any time of year.
All songs have to evoke an emotion, and it's the job of the lyric to explain why the singer is feeling it. In pop songs, the reason should be one that pretty much anybody can relate to. This is why there are so many songs about falling in and out of love: both invoke intense emotions that everyone's experienced at one time or another. Also notable are uplifting, empowering messages such as how you're better off without him or how real beauty is on the inside.
One of the most important aspects of a pop song is that the main melody has to be catchy. Not just pleasant, but the kind of thing you will hum in the shower that night, whether you want to or not. The main melody is a meme that crawls into your ear and convinces you to infect other people by humming it at random intervals, when everyone around you least suspects it. If you spend all day trying to write a lead melody for a pop song's chorus, and you don't catch yourself humming it while cooking dinner afterwards, then improve it or start again until you do.
Structure is how you put the building blocks of verses and choruses together. There are no hard and fast rules when it comes to the structure of popular songs. The vast majority of pop songs, on the other hand, all have one basic structure: intro, verse, chorus, verse, chorus, bridge, chorus, chorus, outro. The chorus can optionally have a pre-chorus before it, to ease the listener into it. You may even get away with sneaking in another copy of the chorus before the first verse, but really, those are about all your options.
The length of a pop song should be between three and four minutes. I'd advise aiming for about three minutes and twenty seconds, which neatly adds up to a forty minute album consisting of twelve songs, or an hour long album consisting of eighteen.
If you had to sum up the aesthetic of pop music in one word, you could do far worse than the word assimilation. Pop Will Eat Itself knew the score: sample it, loop it, fuck it and eat it. If it sounds good, use it. Don't worry about context or connotations. This is pop's greatest strength or its greatest weakness, depending on your artistic point of view.
This sense of fashion, combined with technology enabling ever better production values, is why a contemporary pop song will actually sound contemporary. While almost every genre is defined by its style, namely the choice of instruments and the style of playing them, pop is conversely defined by its lack of one, by its need to borrow from every other genre the same way fashions in other industries do. It's a collage of whatever works.
To sum up, pop songs are songs that the creators want to be as popular as possible, even if that means compromising artistic integrity. When this is done well, the songs can still be good, as the work of several pop producers can attest to. Keeping your ideas within the confines of pop's structure while spurring yourself on to dream up sufficient hooks may even prove to be a fun and worthwhile challenge.
If you're serious about making pop music, I'd recommend Jason Blume's 6 Steps to Songwriting Success, which pretty much tells you all you need to know. If you're not serious, I'd recommend The Justified Ancients of Mu Mu's The Manual (How to Have a Number One the Easy Way), which is much more tongue-in-cheek yet still has a few insights.