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The term blue straggler (BSS or BS) refers to a star appearing in a globular cluster's HRD that does not adhere to the HRD's turn-off point: it is a main sequence star hotter and more luminous than the general run of such stars in the cluster, but (as suggested by the word straggler) there are fewer than one would find in a younger cluster. There is still a question as to how the blue stragglers came to be there. It is generally held probable that they result from interactions between stars, such as binary stars. Stars might actually merge, or mass could be transferred from one star to the other, significantly increasing its luminosity later in its life.
The terms yellow straggler (YSS or YS) and red straggler (RSS, RS) refer to other groupings of stars not in the usual places in stellar cluster HRDs, also in the vicinity of the turn-off point. One interpretation is they are former blue stragglers on their way to the red-giant branch.