SEO (Search Engine Optimization) is the art (some would call it a 'science') of getting organic traffic from search engines. It usually entails publishing content, optimizing the content to rank for certain keywords and building backlinks.
For example if I want to rank for and get traffic when people search for "ecommerce software reviews" I would produce content that is relevant for that keyword and use the best practices Google advises site owners to use. If my content is good, people will start linking to it and over time I'll start getting 'free' traffic from Google and other search engines.
That is basics of SEO but it is a really advanced topic. If you want to learn more I'd recommend downloading Moz's guide here:
Online marketing on the other hand is a much broader topic that includes SEO as well as things like PPC (Pay Per Click), Affiliate Marketing, Blogging, Re-Targeting, Conversion Rate Optimization (my field) and so on. Basically anything involved in selling stuff online, including digital goods and information products. Your stated interest in CPA (Cost Per Acquisition) is part of online marketing.
"CPA" is a metric (the dollar cost of obtaining a sale, download, lead) but is often used interchangeably with "CPA offers" i.e. affiliate marketing of CPA offers.
You can learn more about affiliate marketing here: