What is a CDN and how can it help SEO?
A CDN is a 'Content Delivery Network'. In essence it sits invisibly between your site and the visitor to make the site potentially load faster and protect it from attack.
How does a CDN Work?
A CDN is set up to handle your domain traffic. When someone visits your site they are seeing a 'copy' of your site which is exactly the same as your live site. The CDN will take a snapshot of your site and display the cache to the site visitor. They won't know any different and your analytics will still report traffic as normal. Here is a more technical answer.
How does a CDN help my SEO?
Because the CDN is cacheing your site it can provide a quicker load time of your page which will help SEO. Some providers including Cloudflare offer tools to optimise images and content to make the load time even quicker. These changes are made at the CDN level, not your website and can help optimise the site for non techies.
Who are the major CDN providers?
Cloudflare is a good place to start. Their dashboard and tools are easy to understand, plus they have some basic free services that can help you.
Max CDN offers multi location networks which could help with your international SEO. They have prices starting from under ten dollars a month.
Akami (expensive and used by many global sites.)
One day we may be big enough for Akami. For now, we can just press our noses against the glass :-)
Are there alternatives to CDN?
For SEO benefit you could look at tools for cacheing your site. W3 Total Cache and WP Super Cache both offer Wordpress cacheing to speed your site up. Unlike the CDNs, these won't be help protect your site, just speed it up a little.