I bet youre asking because thats how textbooks show them. Asking whether the simple picture matches reality is a sign of good scientific thinking.

The truth is that cells can be all sorts of shapes, it all depends on what they do. For example, nerve cells (neurons) need to connect with each other, so they have long arms (axons) on one end for sending messages, and arms on the other side (dendrites) for picking up the messages from other cells. Muscle cells are long and stringy and contract to pull on your bones or other muscle. The cells in your small intestine have little finger-like projections on them so that they can absorb over a larger surface. This site shows some animal cell pictures:

So why do they all look round in the book?
Imagine that you had to draw one animal to represent all animals: worms, insects, birds, mammals, fish, etc? What would you draw? Any drawing would be wrong because it couldnt look like a frog at the same time it looked like a jellyfish. Thats the problem that textbook artists have when trying to draw one cell to represent all animal or plant cells.

Plant cells are not all square either. Their shapes also depend on their jobs. There are some pictures of plant cell types at:

Plant and animal cells are different. Animal cells have soft, flexible membranes. Plant cells have them too, but they are inside a tough plant wall that gives the cells their shapes. Plant cells may have pigments like chlorophyll so that they can do photosynthesis. Plant cells can also have a big central vacuole, which is sort of like a big storage bag of fluid in the middle of the cell. Plants can use it to hold wastes or things they need.

Do all one-celled organisms have the same shape? Why do you think this?

No- both come in a variety of shapes. There are round plant cells (those containing starch in a potato for example, or the sclereids that make a pear "crunchy). Both animal and plant cells can be very elongate (Nerves in animals, fibers and conducting cells in plants) and some cells can be highly branched (again nerve cells in animals, and various hairs and sclereids in plants.

For images of plant cells do a Google image search on "Sclereid" and you will get a range of images.

Plant cells are not necessarily square, but they due tend to have distinct edges and be somewhat rectangular. This structure is caused by the cell wall which is very rigid and therefore forces the cell to have a defined shape. However, animal cells do not have a cell wall but only the plasma membrane. So, they do not have a defined shape. They are not necessarily round but instead have an irregular shape. Here are some websites with more information about cell walls, plant cells, and some of the main differences between animal and plant cells.

Actually, both plant and animal cells come in many different shapes and sizes depending on the cell's particular function within the organism (see attached).

For example, neurons at the base of your spinal cord can extend all the way to the tip of your big toe (a single cell that's 3+ feet long!). But there's an important difference between plant and animal cell membranes: both animal and plant cells have a flexible, lipid-based, 'plasma membrane,' but plants also have a tough 'cell wall' structure outside plasma membrane (made mostly of the sugar polymers cellulose and pectin). The cell wall makes plant cells look more rigid, and this is why people sometimes draw simplified plant cells as squares or boxes.

Theoretically, no, and there are some exceptions (muscle cells are fibrous), especially in animal cells. However, all plant cells have cell walls, which animal cells never do, and the presence of cell walls is what gives plant cells their characteristic block-like shape, and the absence is why animal cells, in the lack of other factors, do not.

Cells come in all shapes and sizes, serving different functions in animals and plants. The natural tendency for cells, unless being used for a specialized purpose (such as the long stretched out filaments of our neuron cells in our brains), is to take a somewhat spherical,round shape. This is because it minimizes the surface to volume ratio of the cell (in other worlds, it let's the cell expose the least amount of itself to the world around it). Unlike animal cells, plant cells possess a cell wall, a rigid structure of carbohydrates that gives the cells strength and support. This thick coating around the cells, especially when they are pressed together to form a full plant,can certainly look square. But plant cells come in just as much diversity as animal cells, and can take many shapes. But in general,because of the presence of the protective cell wall, plant cells do look more "square" than animal cells.