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The greek symbol "rho" is density (it is next to the g in the equation).
g = acceleration due to gravity = 32.174 ft/s² = 9.806 m/s²
If you know discharge and diameter (or area), go to Q=VA to compute velocity.
The steady state energy equation for a fluid moving in a closed conduit is written between two locations at a distance L apart.  The loss term h_L accounts for all minor (valves, elbows, etc.) and major (pipe friction) losses.  Often, one knows the flowrate and pipe diameter (or duct area) and is interested in determining the downstream pressure after a certain length of pipe.  All of the losses would be solved for (minor and major), then the energy equation would be used to solve for the unknown pressure or whatever is unknown.  Major losses can be computed using either the Darcy-Weisbach method or Hazen-Williams method.  See Discussion and References for further information.

The energy equation is also valid for open channel flows.

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