The artistry of container composition
Formal balance is when both sides of a composition are equal. A container with classic formal balance may have a tall shape in the center with the plants that surround it forming mirror images of each other. For example, you might use a group of three Asiatic lilies as your focal point, with six ferns - three on each side.
Informal balance is a bit trickier. Rather than identical plants to surround your focal point, you use different plants of a similar weight and texture so that all sides of the composition seem anchored firmly. You don't want one side to be visually lighter than the other. Here you might surround that same lily with ferns on one side, and astilbe of a similar size on the other.
Proportion simply means that the sizes relate to each other in scale. A huge central plant surrounded by tiny, fine textured plants would be out of scale unless something of a medium weight and size intervened to balance them out. A lily surrounded only by trailing ivy would be awkward, proportionally without a fern or some other mid-sized plant to bridge the gap.