For plugin development in WordPress 5, is a block the same as a plugin today?
No, blocks are an entirely different thing. Blocks are blocks of content. The closest thing to them at the moment are shortcodes and possibly widgets.
Plugins are PHP files with a comment at the top of the main file that contains the phrase Plugin Name:. They allow WP to load bundles of code.
Case in point, if you remove plugins, how would you add custom blocks?
If I were going to create a todo list, for example, is that now called a block?
There’s not enough information to define this question, you could implement a todo list block yes. You could use shortcodes, post types, you could do lots of things. None of that changes
I would suggest installing the Gutenberg editor and testing it out. You will quickly see that all the existing methods should still work. You can also attempt to build a simple block. Now is the time to test and give feedback, once Gutenberg is merged and 5.0 is released it will be too late to change how development works fundamentally due to backwards compatibility