SAP Billing Vs Pricing

Hi

We are evaluating third part pricing and biling tool. Can anyone help me understand the following.

1.What is differnce between between Pricing & Billing in SAP world.

2. What are the avaiable tool provide by SAP for Pricing and Billing.

3. Since SAP ECC-SD-Pricing, what business requirement would required third pary pricing tool.

thanks