Many in the blogger community have made the move to ConvertKit in the past year. One of the biggest shocks of the switch was moving from a list-centric way of emailing to a subscriber-centric way of communicating.
List-centric is where we organize everything by lists. We have one master list of everyone who subscribed and we manually move people to new lists based on the information we have on them. For example, we might add someone to a list based on their geographic location or based on what product they bought from us. And in this list-centric view, we're often billed based on the total of our list sizes (instead of our total number of subscribers, which would be far less).
Subscriber-centric, as ConvertKit calls it, is focused on the subscriber. So instead of creating lists and segmenting lists the traditional way of uploading a subscriber to a list, they use the subscriber forms and tags to categorize people.
Now, if you're a blogger, you already know a lot about tags—you use them every time you publish your blog post. You put in keywords that describe the blog post so that people can better find your content. Well, that works with subscribers, too. You can have as many tags as you want on an individual subscriber, and you can create your sending lists based off of those tag rules.