A lot of trees are killed just so you can buy a house. The amount of paperwork is daunting. The lender will ask you for documentation of employment, income, debts, savings, and the source of your down payment money. To back up your claims, you might have to provide tax returns, paycheck stubs, checking statements, investment records, and (if you already found a home) a copy of the home sale contract.
After you apply, the lender will give you a stack of paperwork. The most important document is called the loan estimate of closing costs. It has to be given or mailed to you within three days of application, and it lists the fees that will be charged in connection to the home sale and the mortgage.
While you are trying to make sense of all the paperwork you received, the lender and various third parties are doing their work behind the scenes.