  The ultimate goal of next generation multiple access (NGMA) is to support
massive terminals and facilitate multiple functionalities over the limited
radio resources of wireless networks in the most efficient manner possible.
However, the random and uncontrollable wireless radio environment is a major
obstacle to realizing this NGMA vision. Given the prominent feature of
achieving 360{\deg} smart radio environment, simultaneously transmitting and
reflecting surfaces (STARS) are emerging as one key enabling technology among
the family of reconfigurable intelligent surfaces for NGMA. This paper provides
a comprehensive overview of the recent research progress of STARS, focusing on
fundamentals, performance analysis, and full-space beamforming design, as well
as promising employments of STARS in NGMA. In particular, we first introduce
the basics of STARS by elaborating on the foundational principles and operating
protocols as well as discussing different STARS categories and prototypes.
Moreover, we systematically survey the existing performance analysis and
beamforming design for STARS-aided wireless communications in terms of diverse
objectives and different mathematical approaches. Given the superiority of
STARS, we further discuss advanced STARS applications as well as the attractive
interplay between STARS and other emerging techniques to motivate future works
for realizing efficient NGMA.
