Paypal integration is a method of taking payments from your website using the paypal service, Paypal already have all the necessary processes in place to take payments from credit cards and bank accounts securely, we utilise these processes to enable you to have a payment process on your website.
Putting a payment button onto your website is simple. You can use the PayPals button factory to create the code, you just need to paste the HTML that you get into your website. You will receive an email from PayPal with the details of the transaction.
Example code for a simple PayPal button
Before using the code above you will need to create a PayPal account and enter your own information into the Value"" fields
Link to help with PayPal Buttons
It is worth reading into how to implement IPN as this is an easy way to get notification back from PayPal when the transaction is complete. You will notice in the code above that there is a name="notify_url" and the address to your IPN page. This is a page that the user does not see and exists to verify the transaction. You can use this page perform database function on your own site if you need to update stock levels or send email notifactions to customers.
PayPal guide to Instant Payment Notification