In 'custom redirect' you can simply redirect the buyer to any website that you wish with only the transaction ID passed as GET variable.
Using 'webhook', you can issue a call to your server with a list of key variables with all transaction details. The data is sent as POST variables.
The Webhook and Custom URLs can be different and you can use both features independently. For custom redirect, only a URL has to be specified in the details, while the webhook feature needs some integration for the seller.