j3270 is a set of Java libraries that uses s3270, exposing its capabilities to software running on the JVM.
The design is layered, instead of having a single "high-level" library for using the tn3270 protocol, j3270 provides a base layer that wraps over s3270 and additional libraries built on top. This reduces the problems of trying to build an entire tn3270 implementation from the ground up and building a complete API on top of it. Anything that can be done using s3270 should be possible to do using the same API in j3270's wrapper layer.