  (ABRIDGED) We present the global results of a large spectroscopic survey
carried out in order to identify z~ 5 Lyman break galaxies (LBGs) across ten
widely-separated fields to I_{AB}=26.3. The redshifts of seventy 4.6<z<5.6 LBGs
were identified. We find no significant difference in the frequency of high
equivalent-width line emitters between z~3 and our z~5 samples. The rest-frame
UV continuum slope of a typical z~5 line-emitting galaxy is bluer than that of
a typical break-only galaxy, a difference that is difficult to explain purely
by differences in the ages of their stellar populations. Variation in
metallicity and/or dust extinction can more straightforwardly account for this
difference. Given their UV-continuum slopes, the typical z~5 LBGs have
metallicities a factor of three lower than those of LBGs at z~3. HST imaging
indicates that a large majority of the spectroscopically-confirmed LBGs in our
sample are members of multiple systems and/or show disturbed morphology. Using
local LBG analogues as a model, this multiplicity could be explained either by
super-starburst regions within a larger unseen structure, or by a high
incidence of merging events at this epoch. The current data cannot distinguish
between these two possibilities. The surface density of z~5 LBGs in two fields
is considerably higher than in the rest. Both show clear spikes in their
redshift distributions indicating strong three-dimensional clustering. Neither
structure can be bound given their depth in redshift and probably extend beyond
the observed fields. The three-dimensional distances between LBGs in the
structures are too large for them to have triggered their starbursts through
mutual gravitational interaction, and so it is likely that the short-lived LBGs
represent only a small fraction of the baryons in the structures.
