The joints used to connect the vertical components of cases, cabinets and furniture require strength and often a measure of ease of assembly. The mating edges are long grain to long grain and are therefore present the strongest joints. And since the grain of the mated surfaces run parallel to one another, there is no worry regarding cross grain joint instability.
This joint is the simplest to form, since all you require are two pieces that are butted together. If the exposed end grain contrasts significantly with the exposed face of the glued-on piece, the appearance may suffer, but for most woods this will not be much of an issue.
Care must be taken to carefully align the two pieces, since any misalignment or warping in the woods can reveal a slight lip along the edge. This can either be planed or sanded off, or it can be hidden by adding a v-groove created by adding a bevel to the edge of the two pieces.
Fasteners can be used instead of, or in addition to, glue. Clamping may only be required until the fasteners are attached, after which the clamps can be removed. The fasteners will provide enough clamping force to hold the joint until the glue, if applied, can cure.
Note that for most solid wood joints, mechanical fasteners are not required.
Biscuits offer the benefit of aligning pieces during the assembly process. Well aligned biscuit slots can produce a project that is quick and easy to assemble. Mis-aligned slots, however, can lead to raised edges that may be troublesome to remove or sand smooth.
Once again, please note that biscuits do not offer any mechanical strength. Their only purpose is to aid in the assembly of the project. In fact, biscuits are weaker than most woods they are used in joining.
A splined joint is essentially a full-length biscuit. Both parts have a groove that accepts a spline. Variations are a through spline, where the spline is visible when viewed from the end of the wood, or a blind spline, where the spline slot has been cut short, and no spline is visible from the end of the joint.
Splines do not offer much strength, however they can be a great aid in speeding the assembly of a project.