  Many stars do not live alone, but instead have one or more stellar
companions. Observations show that these binaries, triples and higher-order
multiples are common. Whereas the evolution of single stars and binaries have
been studied extensively, the same is not true for the evolution of stellar
triples. To fill this gap in our general understanding of stellar lives, we aim
to systematically explore the long-term evolution of triples and to map out the
most common evolutionary pathways that triples go through. We quantitatively
study how triples evolve, which processes are most relevant, and how this
differs from binary evolution. We simulate the evolution of several large
populations of triples with a population synthesis approach. We make use of the
triple evolution code TRES to simulate the evolution of each triple in a
consistent way; including three-body dynamics (based on the secular approach),
stellar evolution and their mutual influences. We simulate the evolution of the
system up until mass transfer starts, the system becomes dynamically unstable,
or a Hubble time has passed. We find that stellar interactions are common in
triples. Compared to a binary population, we find that the fraction of systems
that can undergo mass transfer is about 2 to 3 times larger in triples.
Moreover, whereas in binaries the orbits typically reach circularisation before
Roche-lobe overflow, this is not true anymore in triples. In our simulations,
about 40% of systems retain an eccentric orbit. Additionally, we discuss
various channels of triple evolution in detail such as those where the
secondary or the tertiary is the first star to initiate a mass transfer event.
