The difference between SEO and PPC is fairly simple.

SEO = Unpaid or ‘organic’ traffic from search engines.
PPC = Paid traffic from advertisements displayed on search engines.

One main difference between them that has emerged over time is that search engines tend to give a lot more flexibility to change the appearance and utility of the paid listings versus a more standardized and rarely changing appearance / utility for the organic listings.

Ads on search engines are often skipped over by consumers or blocked, others are clicked by consumers on accident or by mistaking them as the organic search results they are using an engine to get.

Both have value to drive increased revenue and most SEO and PPC consultants recommend a mixture of the two for best results. You can combine both under the same agency or you can use different consultants/agencies for each one.