So, what's the difference between crystalloids and colloids?

CRYSTALLOID SOLUTIONS

Type pH Tonicity Components
Replacement Fluids:
0.9% NaCl 5.7 Isotonic Contains only Na, Cl, and water
Balanced Electrolyte Solution (Norm-R) 7.4 Isotonic Contains K, may contain Ca,Mg
Specialty Fluids (NOT for fluid resuscitation!):
D5W 5.0 Isotonic 5% Dextrose in water

COLLOID SOLUTIONS

Type Physiological Objective
Synthetics:
Hetastarch, Dextran Provides oncotic pressure (tends to stay in the vascular space)
Natural:
Red Blood Cells Provide oxygen carrying capacity
Fresh or Fresh Frozen Plasma Provides oncotic pressure (tends to stay in the vascular space) as well as clotting factors and other essential proteins
Albumin Provides oncotic pressure (tends to stay in the vascular space)
Courtesy of Dr. Cynthia Otto, University of Pennsylvania School of Veterinary Medicine.

Both crystalloids and colloids increase intestinal blood flow and systemic arterial pressure; however, colloids may have a longer duration of effect.  Colloids also result in a net movement of fluid from the intestinal lumen to the blood, whereas crystalloids can exacerbate transmucosal fluid movement into the intestinal lumen.