The booking deposit secures the property for a number of weeks (usually three) to give you time to get finance and contracts sorted out. The booking deposit is deducted from the purchase price.
The contract deposit is paid at the same time that the contracts are signed. It's usually 10% but usually the booking deposit is taken out of this amount. (So contract deposit is usually 10% - booking deposit).
If you have a 100% mortgage with no savings to cover the contract deposit, you may need to get stage payments for your mortgage.
The other 90% isn't due until completion, and depending on the house in question, there could be a few months between exchange of contracts and completion.