A common way to design container gardens is by using plants designated as thriller, filler and spiller plants. Thinking of container plants this way is a particularly helpful to beginners. It also a great way for anyone to get started in designing exciting plant combinations without getting intimidated by all the choices out there.
Thrillers are generally taller and often used in the center of this kind of design, as well as being thought of as the most exciting plant.
The fillers are the plant that fill in the middle, between the pot and the thriller, often a mounding plant that adds texture. The spillers, 'spill' or cascade over the sides of the pot. They are usually placed closest to the rim of the container so they will tumble out of the pot. Spillers can be flowering or foliage plants and can be used to add a pop of color or a texture that can be a counterpoint to both the filler and thriller plants.
To my mind spillers can be as thrilling as thrillers and can even be used in a pot by themselves. There are huge numbers of plants that can be used as spillers. Some are annuals, some perennial, some are chosen for their foliage and texture, others for flowers and color. Some spillers, like oregano or some types of thyme, are edible.
Sweet potato vines are classic spillers. They come in a wide array of leaf shapes and sizes and colors from bright to almost black.
Another common flowering spiller is lobelia. Its frothy blue flowers add both color and texture.
Ivies make great spillers. They are a somewhat structured plant and can be used to give texture, draping down the side of a pot.
While you can use one variety as a spiller throughout your pot, it is often effective to alternate, two or if your pot is large, even three varieties of spiller plants.
Also, some plants can be both fillers and spillers--if they are mounding and trailing--verbena, bacopa, oregano, and some varieties of sedum, to name just a few.
Here are some examples of foliage spillers:
Here are some flowering spiller plants:
Definition and Suggestions for Thriller Plants
Definition and Suggestions for Filler Plants
While the design concept of thriller, filler, spiller can be helpful, it can also lead to container gardens that all look the same--that one tall plant in the middle, surrounded by fluff and then something spilling. If you are doing a group of containers, I would caution against doing them all using this technique.
Instead, you can thing of the group of containers using this theory, placing
the grandest and tallest container towards the center-back of the group, the middle sized, containers surrounding the large and the smaller containers, mainly with spiller plants closest to the ground. I've seen this used to great affect when grouping containers.