This is a nice paragraph from link below which sums it up very clearly.

Windows as a service: Simplified and Aligned Windows for IT Pros

As part of the alignment with Windows 10 and Office 365 ProPlus, we are also adopting common terminology to make it as easy as possible to understand the servicing process. The two most important terms to understand:

Semi-Annual Channel. These are the twice-per-year feature update releases, targeting March and September, designed for the broad population of general-purpose PCs used throughout organizations. Each of these releases will be serviced for 18 months from the date of release. (The Semi-Annual Channel replaces the Current Branch [CB] and Current Branch for Business [CBB] concepts.)
Long-Term Servicing Channel. These are less frequent releases, expected every 2-3 years (with the next one expected in 2019), designed for special-purpose PCs such as those used in point-of-sale systems or controlling factory or medical equipment. Each of these releases will be serviced for 10 years from the date of release. (The Long-Term Servicing Channel replaces the Long-Term Servicing Branch [LTSB].)