Fluid mechanics is a small subject wearing a large vocabulary. Before any relation is worth memorising, you have to be able to look at a number on a commissioning sheet and say what KIND of thing it is. The unit does that for you every time, so learn to read the unit first and think second.

Density is how much mass a volume holds: $\rho = \dfrac{m}{V}$ — read aloud rho equals m over V, with $\rho$ the Greek letter rho. $m$ is the mass in kilograms, $V$ is the volume in cubic metres, and $\rho$ comes out in kg/m³. Water is 1000 kg/m³, near enough, and that single number anchors the whole subject. Specific gravity is that same density measured on water's scale: $SG = \dfrac{\rho}{\rho_{water}}$, where $\rho_{water}$ is 1000 kg/m³. It carries no unit at all — a density over a density, everything cancelling — which is exactly why the trade quotes it: SG 0.85 means the same thing to a technician in Halifax and one in Houston, whatever units their gauges wear.

Pressure is a force spread over an area, in pascals — one newton per square metre. That is a tiny pressure, so plant rooms run in kilopascals and the American trade runs in psi, pounds per square inch. Head is the same pressure written as a height of fluid, in metres — or in the US, feet of head. And flow rate is a volume per unit time: L/s, m³/h, or the trade's own GPM, gallons per minute.

One nugget to carry out of here. Pressure and head describe the same physical fact, but a pump curve is drawn in HEAD rather than pressure for a reason: the head a pump makes barely changes when you switch fluids, while the pressure it makes changes in exact proportion to the density. Head is the honest description of what the impeller does; pressure is what the fluid does about it.