If you want to add PayPal to your website you can choose from different solutions like Payment Standards (the classic html buttons) and ExpressCheckout, a little bit more advanced solution that requires some integration steps. Both solutions accept PayPal account payments and can give the option to accept card payments (where available). If you are using a third party solution as eCommerce (like Magento, Prestasho, WordPress, Shopify etc..) then you should check an available module to enable/install in your eCommerce and just configure it (ask directly to your eCommerce provider how to configure it as it is something customized).
You can refer to the technical and overview guide at the links below:
Payment Standards
ExpressCheckout Classic
ExpressCheckout JS4
As well, you can eventually check for PayPal Pro that is designed to mainly accept card payments but only available in some countries (Uk, Us and Canada): link
You may want to check as well REST APIs
The most recent integrations use Braintree, a PayPal company, and you can check directly with Braintree how to integrate a payment solution for your website: Braintree Overview
