The lineage that gives rise to reproductive cells transmitting genetic information between generations, distinguished from somatic body-cell lineages.
In animals with an early segregated germ line, only selected cellular descendants normally contribute to gametes. In other organisms, reproductive lineages can be specified later. A mutation’s presence in a body cell therefore does not alone establish transmission to offspring. [ref]
Mechanisms protecting the inherited genome from transposable elements and other threats. Suppression of transposon RNA, chromatin repression and removal of damaged cells are different protective strategies.
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It is quite mind blowing when you think about it, that the huge majority of your body's cells is essentially just there to support a tiny ammount of germline, which are the only cells that can actually pass on! It is fun to imagine the cell type tree for this, with a huge branching of somatic cells, and only a few germline going forward.