Edwin Hubble's discovery of cosmic expansion proved that motion was truly 
universal in its scope.  Mathematically, that expansion can be expression in an 
equation known as Hubble's Law:

v = Hr       (recessional velocity = the Hubble constant * distance)

"H" is the constant of proportionality, which astronomers have named
the Hubble constant in Edwin's honor. 

As a simple exercise, if H = 70 km/s per Mpc, and you measured a galaxy moving 
away from Earth at v = 350 km/s, then the distance
to the galaxy would be r, where  

350 km/s = (70 km/s/Mpc) * r   and thus r = 5 Mpc.

Now, if we realize that both km and Mpc are measurements of length, this actually 
gives us a way to compute the approximate age of the universe!  The quantity
1/H - the multiplicative inverse of the Hubble constant - can be expressed in 
units of time, and is roughly the amount of time the universe has been expanding.