  Gravitational waves (GWs) are fluctuations in the fabric of spacetime
predicted by Einstein's theory of general relativity. Using a collection of
millisecond pulsars as high-precision clocks, the nanohertz band of this
radiation is likely to be directly detected within the next decade.
Nanohertz-frequency GWs are expected to be emitted by mergers of massive black
hole binary systems, and potentially also by cosmic strings or superstrings
formed in the early Universe. Direct detection of GWs will open a new window to
the Universe, and provide astrophysical information inaccessible via
electromagnetic observations. In this paper, we describe the potential sources
of low-frequency GWs and the current status and key advances needed for the
detection and exploitation of GWs through pulsar timing.
