I could write essays on the subject - some background, I've been surrounded by music my whole life, learned various instruments, my dad owned a recording studio (local, smaller bands) and I'm an amateur musician, songwriter, engineer, producer and the like...I'm don't claim to be a guru but I've spent decades learning all about the processes involved.
In a rock context, each band usually has main songwriter, who will come in with typically a chord progression, riff and/or melody. Different bands will deal with it differently...Queen/U2 moved on to a system of all the group being credited as songwriters - this is partly down to giving the whole group equal royalties and recognising that the whole group contributes (also to save ego battles). The Beatles is almost all 'Lennon/McCartney' even when one would barely contribute to a particular song - you could argue also that even though JL/PM may have written a chord progression to credit themselves, a Harrison riff might actually be what the listener identifies with more, so a lot of people who contribute to a song may not be credited...on the other hand a lot of modern 'artists' walk in to a music studio with everything already done but take a songwriting credit because it suits everyone for a 'star' to push the song sales...I really wouldn't trust any of them...in fact there was a good article about mixing credits for Kanye, which I'll post a link to at the end.
Traditional bands are more capable of writing chords, melodies, riffs etc don't generally need any help. By all accounts Kanye has no musical training or knowledge - he's someone that you're told knows what he wants but isn't capable of doing it himself. To me, this immediately makes 'his' music and claim to be a 'genius' immediately flawed but I digress. Anyway, as this is the case, he needs massive amounts of help, hence the huge credits. He essentially uses multiple people to contribute, picks what he likes and takes a lot of the credit - he's more an A&R man like Simon Cowell than a musician.
There seems to be a lot of confusion about what a 'producer' does. It varies but usually they will be responsible for capturing the sound a group wants to achieve, sometimes being part of the creative process but other times not. What Kanye does is farm out all these responsibilities - he's almost a project manager, pulling other people's work together to complete a project. There's a talent in it but it's massively overstated. He may do some beats but it's hardly like writing the Bohemian Rhapsody chord progression - he also sends songs to multiple mix engineers (mixing is an art form that I could write pages about and IMO is harder and more important than the recording). I really can't stand Kanye West and his music - if people do, it's fine but stop calling him an "amazing producer" without knowing what it is.
This article about Daft Punk's Random Access Memories is a great (but long and technical) read:
http://www.soundonsound.com/sos/jul13/articles/daft-punk.htm
And this is the Kanye article:
http://www.soundonsound.com/sos/feb06/articles/bauer.htm
