Think of your three closest friends. Now answer these questions: What did their parents do for a living? Did they have a pet when growing up and if so, what was it called? What is their favourite film?
The chances are you're struggling (unless you're 15, in which case you probably got a full house). Why? You don't need to know this information to be friends. It's just the same when we write characters.
I've just finished reading One Day by David Nicholls and it's startling the lack of hard information we know about the characters throughout. In the opening pages - we only discover their names on the 3rd page by the way - we very quickly get an impression of who the characters really are, through knowing their thoughts and attitudes. For example, " 'I think reality is over-rated,' he said in the hope that this might come across as dark and charismatic." We know immediately he is young and not as confident as he might appear to be.
This is much more interesting than knowing, for example, where he was born or what his father did for a living. And it also reflects real life. You are far more likely to know your friend's attitudes to life than you are to know facts about their past history. So, the reader needs to know about characters' attitudes to life, but not necessarily facts about their past history.
Secondly, how did you learn about your friend's attitudes to life? You probably knew little bits straight away from how they spoke and dressed, a few more from what they said on that first meeting. Then, each time you met up you learned a little bit more about what made them tick. You might have had a long heart to heart conversation at some point, but it's unlikely that happened on your first meeting, and even more unlikely that it happens every time you meet up with your friend. This is exactly the same as when you're writing. You want to drip feed information to the reader so they gradually build up a picture of your characters.
Finally, have you ever been to a party where you've met someone who seems on a mission to fill you in on the most interesting topic in the world: them? I've been trapped by someone like this several times in my life. They tell you about themselves in exhaustive detail while you stand there glazing over and hoping you'll be able escape soon. People like this are bores. Well, guess what - so are characters who you know everything about when they first turn up.
One Day is a good example of information being carefully rationed, and the gradual release of information about the characters is one of the factors that have made it such a success. You really don't need that much backstory information to hook a reader into your characters. Concentrate on making them interesting, not the facts about them.