All liquids are fluids, but interestingly, not all fluids are liquids. Anything that can flow -- such as a gas -- is a fluid, and can create buoyant force. Buoyancy is caused when areas of higher pressure beneath an object exert force upward toward areas of lower pressure. The amount of buoyant force that a fluid exerts, however, is determined by the object's volume and according to Archimedes' principle.
Pascal and Pressure
Before you can understand how differences in fluid pressure can affect buoyancy, you need first to understand how pressure behaves in fluids. Pascal's principle states that when pressure is changed at any location within a closed system, that pressure change will be felt equally at every point within that system and in all directions. This principle is that which allows hydraulic systems to function. It also dictates that within a body of fluid where there aren't any additional factors that affect pressure, the pressure will remain constant and even. On Earth, however, there is usually at least one other force that causes a variance in the pressure of a fluid, and that force is gravity.
Depth and Difference
Gravity pulls downward on everything that has mass. Therefore, when gravity pulls downward on a body of fluid, the weight of the fluid in the upper parts of the body pile upon the the fluid in the lower parts, creating a grade of increasing pressure as you move downward within that fluid. For example, if you dive deep into a lake, you will feel increasing pressure in your ears -- and perhaps even against your body -- the deeper you dive. If you stop swimming downward, the higher pressure below you will push you back up toward the area of lower pressure. In this way gravity has created a pressure dynamic which dictates that there will always be greater pressure beneath a submerged object than above it.
Archimedes and Amount
The Greek philosopher and mathematician Archimedes took this understanding of pressure one step further, and made sense out of why a fluid applies a certain amount of upward force to an object and causes it either to rise and float or allows it to sink. He determined that the upward force was equal to the weight of the water displaced by the submerged object. For example, water weighs one gram per cubic centimeters. If you submerge a ball with a volume of 25 cubic centimeters, you will have displaced 25 grams of water. Therefore, the resulting buoyant force on that ball will be 25 Newtons (Newtons are units that measure force). This buoyant force is always based on the mass of the displaced water, however, and not the mass of the object.
Density as Decider
Density is ultimately the factor that determines whether an object will float, sink or remain neutrally buoyant in a fluid. For example, if that 25 cubic centimeter ball is hollow and filled with air, it will be lighter than the 25 grams of water that it has displaced, and will float. If the ball is made of a denser material, such as iron, it might be much heavier and sink quickly to the bottom of the body of water. If you submerge a ball that weighs exactly 25 grams, however, the buoyant force will not drive it up to the surface, but simply keep it from sinking. This ball will will remain neutrally buoyant in the body of the fluid until acted upon by an outside force.