It depends if you have put your bank account on file then yes it is linked to it and will draw the funds that you pay out from there. Unless you have also put a credit card on file and you choose to use this as your funding source.
When you buy an item on ebay the seller may send you an invoice through paypal which you can then follow and pay or from your my ebay area you can click on the pay now option to the far right of the item listed and this will take you to paypal.
If you recieve money into your paypal account thou it isnt automatically paid into your account that if you have on file(if that is the case) as you have to pay a fee to withdraw any funds from your paypal account. Otherwise your funds are left in your paypal account and the next time you pay for something using paypal you can use this as a source of funding.
Hope that helps