I'm not easily finding a confirming link, but your guess is correct - the "Login" licenses are for logins-per-month pricing, and the plain "Community" licenses are for named-user licensing. I just took a training on Communities implementation where this was covered.

As mentioned, these have all been subsumed into Community, the portal to rule them all.

For the curious, here's the lineage.

Self-Service

This was the very first portal. It's main draw is it's FREE. User management is funky (not stored in the User table), and customization is extremely limited.

Partner / Customer Portal

These are the second portal varieties. They're effectively the same in terms of customization and user management, but just targeted at different user types. Partner portal is for your, well, Partners, who need to work with Leads, Contacts, Opportunities, etc. Customer Portal is more focused on support needs, so your customers can interact with cases. Partner licenses, having more access to the core CRM features, were significantly more expensive.

Community

I haven't gotten much chance to try these out, but the high level idea is you no longer have to worry about all the different portal types there is just one.