Normal saline contains roughly the same osmolarity of NaCl as blood. It is a crystalloid because it contains a water-soluble molecule.
If you used a saline solution with a larger concentration of salt then it would act to induce a shift in fluid from the intracellular to the extracellular space due to the osmotic gradient (hypertonic saline is therefore effective at reducing intracranial pressure as it decreases brain water).
Colloids contain larger molecules that are (for the most part) not water-soluble e.g. toothpaste (sorry for the non-medical example, I'm a chemical engineer ^_^).
They are both volume expanders and plasma expanders. More information on the respective advantages / disadvantages of each can be found here:
Hope this helps!