The 100G standard was ratified eight years ago and while market adoption has been slow, the technology is gaining some traction now. There are many reasons for the ebb and flow of technology adoption and in the case of 100G the QSFP28 interface is certainly one.
QSFP stands for QuadSmall Form-factorPluggable and simply put is an industry standard multi-source agreement (MSA) that specifies the form factor and electrical interface for a 100G transceiver. Earlier versions called QSFP and QSFP+ supported 1G and 10G, respectively, while the latest incarnation is for 100G (the 28 refers to the single lane speed of 28 Gbit/s).
QSFP28 is important for 100G adoption for at least three reasons: 1) lowers cost, 2) has a smaller form factor and 3) uses less power. This contrasts with the first 100G MSA called CFP (CForm-factor Pluggable) which yielded bulky transceivers that were relatively power hungry and expensive. Fortunately, CFP is behind us and QSFP28 will be the transceiver that leads the industry into mass 100G adoption.