While there are quite a bunch of them, the fundamental conceptual elements of Kubernetes are fairly accessible. Nodes? Check. Containers? Check. Pods? Check. Services? Pretty straightforward, although there is some not oft mentioned complexity in the underlying network routing across pods.
A Kubernetes Service is an abstraction which defines a logical set of Pods and a policy by which to access them - sometimes called a micro-service. The set of Pods targeted by a Service is (usually) determined by a Label Selector (see below for why you might want a Service without a selector).
Check out the “Virtual IPs and service proxies,” subhead of the Services docs to see what I mean about networking.
Exposing services to the outside world? Not so much. Alternatively, if you can make sense of this gobbledygook, you’re a better person than I. Service Types are the singular concept where I have yet to see a good, comprehensible tutorial, either written, audio, or video. Something I’ll be on the lookout for.