With electro there's a blurring between that and house which will kinda confuse those who aren't as big of music nerds as we are.
It's not complete if you don't include Techno, DnB/Jungle and Ambient Electronic music in the conversation IMO, and more and more Dubstep, even if the term is continuing to get corrupted by Skrillex and his minions.
House/Trance is the still the most obvious mainstream divide, though it's been losing meaning over the past few years as trance has becoming increasingly simple (with the most high profile trance DJs, most obviously Tiesto, going as far as to sell out completely into House trends) and with the growth of progressive house borrowing a lot from trance. If anything the biggest divide nowadays is between purists, IDM experimentation and mainstream celebrity DJs who fully embrace the Americanization of electronic music (or "EDM", ugh)
Not sure I can completely sign that.
If you look at the early electro, it has more roots in hip hop and breakbeat more than anything else. Electro as a genre went also through a great deal of changes, and if you look at where it is now, it bears more resemblance of techno. Such artists as Drexciya, Blackstrobe (inactive, but Ivan Smagghe and Arnaud Rebotini still produce quite a lot of their own stuff), Apparat, Ellen Allien etc. have been, and most likely will be for quite a time to come, a living example of artists combining both techno (using this in a more general term) and electro.
What comes to Swedish House Mafia.... those three enthusiasts decided in late naughties that it might be a riveting experience to slaughter electro and house, and in the aftermath to create their own pet Frankenstein called electro-house. That sickening creation still turns me inside out and makes my ears bleed. They, for me, killed two great genres and then resurrected it as one big piece of failure. The amount of popularity they got through selling out is beyond me, and now every 15 to 20 year old associates either House or Electro with those three little pigs.
Interestingly enough, both Sebastian Ingrosso and Steve Angello had remotely successful careers before going apeshit. Their collaboration, the Sinners, was straightforward tech-house / prog house with a good detroit-ish vibe to it. Steve Angello's releases between 2003 and 2005 were gold, especially his first ever release "Voices" which was an enormous hit in 2003-2004. Then when they were gradually establishing themselves, everything went south. Swedish House Mafia has nothing to do with neither house or electro. It's just puke.
Trance and House were decent on their own as well throughout the nineties, until people the Dutch decided to add their own little flavour to it (Yes, Ferry Corsten, Armin van Buuren and Tiësto - I am talking about you) by ruining the genre completely. It turned from beautiful melodies and smart drum-machine use into over-compressed, super-saw filled and overly sweet blue bloody stilton. I still am gobsmacked by how much people can destroy simple things and turning them into abominations of them earlier selves.
Then indeed came the Americanization of electronic music, mainly cheesy trance and overly shit "house" and "dubstep" produced by the above praised Swedish House Mafia, Tiësto and Skrillex. The whole thing just exploded and a new market, for overly produced crap with little similarities to the original genres, opened. Now every teenager with glow sticks thinks Tiësto's the best thing ever happened to trance. WRONG!!!
I still think of those listening to trance as sort of simpletons, musically speaking. That's only due to not being able to understand slightly more difficult and abstract sound patterns. The fact that you don't have to process the music you hear, because it has been made so easy with little snares here and there, repetitive synth key patterns, a strong kick either emanating from Fruity Loops or Logic accompanied by a weak hi-hat and 'beautiful' female vocals, is somewhat boring and far from being creative on any scale.
All in all, electronic music in general is in much weaker state than it was say 10 years ago and it's heading towards utter oblivion. Luckily there still are smaller labels and artists out there that produce high end stuff for the music's sake and not
solely (emphasis added) for their pockets' sake.