Before I reply to Pete...
Fandom for me, as Pete said, is just a word for fans. Fan girls/boys use it as a catch all for fans the like you'll get on Tumblr.
Now there is no problem with 90% of every fandom. That 10% give us a bad name and its unfair because these bad names stick and it gets annoying and ends with people who are usually balanced and fair to get pissed and annoyed at people.
For me recently this has all been shown on Tumblr. I follow a lot of confession blogs because I find them interesting. Recently I've seen Potter fans complaining about SuperWhoLock and how they are the "older" and "bigger" fandom and they are fed up of these "new" fandoms showing up and stealing their fandom. Well I'm sorry Potter fans Whovians have been around for 50 years and don't get me started on Trekkies! Fandom didn't start with Potterheads. But similary I'm fed up of this mashing of fandoms, SuperWhoLock... Well yeah I like Doctor Who and Sherlock and they fit as British dramas made by the same people but were does Supernatural fit in?
Obviously the thing you'll find with fandoms is that they like the same general things you do so that is how SuperWhoLock was born, to be honest I say I'm fed up but I'm not really...
Its just one of those things really. Fandom will give itself a bad name, I was reading something about Christopher Tolkien not liking LotR fandom (though he seems a miserable git anyway) because he's seen some of the fans and think the message of the books have been lost. That isn't true, all fandoms have very talented and imaginative people in them and they work hard making wonderful things out of the media that means most to them. Some of the bad name that fandom has is just because they are passionate about something which is totally horrible!
For me I'm in a few fandoms. F1, Harry Potter, Sherlock, Doctor Who, Tom Hiddleston, Marvel.... We all argue, we all have different opinions and sometimes I sit there in total amazement at the small mindedness of some of those 10% but most of the time I laugh, I cry and I'm in awe of the talent.
I mean I spent 10 minutes reading an ongoing debate on whose bum a screencap is. Heck you just have to see some of the amazing things that Hiddleston fans have done to see how amazing fandoms can be, look at GPD and his charity raising.
Fandom doesn't deserve a bad name, everyone and everything has bad areas and everyone and everything has good areas you can't tar us with one brush.