If not for Michael Jackson stealing her thunder a year earlier with Thriller, Madonna would be seen as the divisive artist of the 80s, the point at which pop music irrevocably shifted into something entirely new, with the starting point being her
'83 album, Like A Virgin. If Jackson revived and justified disco, merging it with traditional R&B and jazz via producer Quincy Jones in the process to create the foundation for New Jack Swing and contemporary R&B, Madonna did the same thing for kitchy-synthpop and New Wave bands from the late-70s/early-80s.
Unlike Jackson, whose R&B meets disco sound is still novel to this day, Madonna is much more easily described in terms of modern groups. It's apparent from a quick glance at female pop stars that Madonna's shadow is nigh-inescapable. Britney Spears, Christina Aguilera, Lady Gaga (especially Lady Gaga), and any other sexpot pop star can trace their inspiration and style straight back to Ms. Ciccone.
Madonna's career can be rather neatly split up into three periods, largely defined by the decade in which certain albums were released. Make no mistake; it's all pop music through sheer name-power alone. But each of these periods does have some distinct differences.
The Immaculate-period (early-80s), named after her greatest hits compilation for the 80s, The Immaculate Collection, is comprised of her first three albums, and is defined by singles like "Holiday", "Lucky Star", and "Like A Virgin". Despite the occasionally dark (or at least surprising) lyrical content ("Papa Don't Preach", a positive perspective on abortion) this is her most 'poppy' period, in line with the synthpop that came before and after it. This is the period that Cyndi Lauper and early Britney Spears shamelessly aped.
The Erotica-period (late-80s, early/mid-90s), named after the Erotica album, involved... well erotica. This is the period containing Madonna's raciest videos (such as her virtually-topless appearance in "Vogue" or "Human Nature"'s psuedo-S&M) and the notorious 'Sex' book. Even if her record sales did take a hit during this period, they were so high to begin with that it's hard to really notice. The albums released during this time-frame were generally moodier and closer to trance than the straight synthpop of her earlier albums. This move is best demonstrated by the last album of the period, Ray Of Light, which was musically built entirely within ProTools and effectively marked the end of Madonna's sexpot image. I mean, everyone still wants to sleep with her, but she isn't shoving it in our faces as much anymore.
Most of the time anyway.
Her current period, which has been going on since 2000's creatively titledMusic, is here being termed the Hard Candy-period, after her most recent studio album. As suggested by 05's Confessions On The Dance Floor, the 00s have seen Madonna backpedal from her more atmospheric releases in the late-90s to straight up dance floor hits in the style of "Vogue". I don't have much to say about this period because I have a freakish loathing of modern day dance music, no doubt caused by my intense, pathological hatred of will.i.am and "In Da Club" (which I still contend is the worst best-selling single of all time, tied only with "Boom Boom Pow" and "I'm Yours").
The key thing to note about this current period is that Madonna is still effortlessly keeping pace with the currents of pop music. While many artists lose track of the generational pulse (see: KISS and Oasis, the latter of which did it in record time) or simply decide to chuck all caution to the wind and doing something arty (see: The Beatles), Madonna has stayed comfortably on the groove, so to speak. I'd gripe about it being lazy and uninspired, but I can't exactly claim that making pop music is brainless work.
I'd be more inclined toward bitchiness if Madonna was one of those Tin Pan Alley manufactured cutouts like dear old Britney, but I have to give the woman credit where credit is due; she's written or co-written nearly all of her biggest singles, and is the second best-selling female artist of all time (right beneath Barbara Streisand; go figure). As Michael Jackson is to modern hip-hop and R&B, Madonna is to the modern dance floor.