  [Abridged] The [CII] 158um line is an important tool for understanding the
life cycle of interstellar matter. Ionized carbon is present in a variety of
phases of the interstellar medium, including the diffuse ionized medium, warm
and cold atomic clouds, clouds in transition from atomic to molecular, and
dense and warm photon dominated regions (PDRs). The Galactic Observations of
Terahertz C+ (GOTC+) project surveys the [CII] line over the entire Galactic
disk with velocity-resolved observations using the Herschel/HIFI instrument. We
present the first longitude-velocity maps of the [CII] emission for Galactic
latitudes b=0deg, +-0.5deg, and +-1.0deg. [CII] emission is mostly associated
with spiral arms, mainly emerging from Galactocentric distances between 4 and
10 kpc. We estimate that most of the observed [CII] emission is produced by
dense PDRs (47%), with smaller contributions from CO-dark H2 gas (28%), cold
atomic gas (21%), and ionized gas (4%). Atomic gas inside the Solar radius is
mostly in the form of cold neutral medium (CNM), while the warm neutral medium
(WNM) gas dominates the outer galaxy. The average fraction of CNM relative to
total atomic gas is 43%. We find that the warm and diffuse CO-dark H2 is
distributed over a larger range of Galactocentric distances (4-11 kpc) than the
cold and dense H2 gas traced by 12CO and 13CO (4-8kpc). The fraction of CO-dark
H2 to total H2 increases with Galactocentric distance, ranging from 20% at 4
kpc to 80% at 10 kpc. On average, CO-dark H2 accounts for 30% of the molecular
mass of the Milky Way. When the CO-dark H2 component is included, the radial
distribution of the CO-to-H2 conversion factor is steeper than that when only
molecular gas traced by CO is considered. Most of the observed [CII] emission
emerging from dense PDRs is associated with modest far-ultraviolet fields in
the range chi0~1-30.
