Crest and Trough
The crest is the highest point of a transverse wave and the trough is the lowest; the distance between successive crests is one wavelength.
Definition
On a transverse wave, the crest is the highest point and the trough is the lowest, taking turns as the wave rolls along, with crests and troughs always the same size, since a wave rises exactly as far above the middle line as it dips below it. A crest is the point of maximum upward displacement, a trough the point of maximum downward displacement; the distance from one crest to the next equals the wavelength, and the height of a crest above equilibrium is the amplitude, so the vertical distance from crest to trough is twice the amplitude. For a wave $y(x,t) = A\sin(kx - \omega t)$, crests occur where the phase $kx - \omega t = \pi/2 + 2\pi n$ and troughs where it equals $3\pi/2 + 2\pi n$, so adjacent crests are separated by $\Delta x = 2\pi/k = \lambda$ and each crest advances at the phase velocity $v = \omega/k$; a particle at a crest has zero transverse velocity and maximum downward acceleration ($-A\omega^2$), and in interference, crest-on-crest is constructive (path difference $n\lambda$) while crest-on-trough is destructive (path difference $(n + \tfrac{1}{2})\lambda$).
Example
A surfer rides the crest of an ocean wave while the low water just before the next wave is the trough; ocean swells with crests $2.4\ \text{m}$ above their troughs and spaced $60\ \text{m}$ apart have amplitude $2.4/2 = 1.2\ \text{m}$ and wavelength $60\ \text{m}$. Two speakers $3.0\ \text{m}$ apart playing $686\ \text{Hz}$ in phase, with $\lambda = 343/686 = 0.5\ \text{m}$, give a listener $4.0\ \text{m}$ from one speaker and $5.0\ \text{m}$ from the other a path difference of $1.0\ \text{m} = 2\lambda$, so crests arrive together and the sound is loud, while a $1.25\ \text{m}$ difference ($2.5\lambda$) puts a crest on a trough and the sound nearly vanishes.
Key Insight
When a crest from one wave meets a trough from another, they cancel, and when two crests meet, they add up to a bigger crest, which is how rogue waves form at sea. The crest is a place, not a thing: water molecules do not travel with the crest, which is simply a moving pattern of which molecules happen to be at maximum displacement at that instant.