Briefly over the past 5 years there has been a surplus of cheap credit on the world money markets.
This has led to lenders giving lots of cheap credit to people who may not in the past have paid it all back, including 100% mortgages to first time buyers. This is risky as it relies on the fact that house prices need to increase rapidly.
The credit crunch is basically that lenders are being more careful with whom they give the money. Hence the withdrawal of most of the 100% and 95% mortgages.
This means thet new buyers need to find 10% of the house price as a deposit. If the house is being bought at 150k a buyer will now have to find 15k to buy it.
More info here.
http://news.bbc.co.uk/1/hi/in_depth/business/2 007/creditcrunch/default.stm