Lines are used to make shapes and divide space. There are many kinds of lines, and some are invisible, yet still important.
shapes and forms are pictorial figures made from lines. Shape and form are synonymous. They're basically just blobs. They may represent real things, or they may be abstract. There are some textbooks and teachers who will argue that shapes are two dimensional (2D) while forms are 3D. I think it's fair to agree they have these associations - sculptors use the word 'form' when casting plaster molds of their work. But you can use the words interchangeably and people will understand you.
Volume, on the other hand, suggests a 3D form - the idea of being able to reach in and grab a solid object. Volume also suggests weight, because solid forms are heavy. A volume isn't a shape. When we talk about a "volume of water" it could have any shape, but the main thing is it's 3D. If a shape or form in a painting "has volume" it appears 3D. If a sculpture has lots of volume, it is 3D - you might think that's obvious, but some sculptures are very thin.
Mass is similar to volume. When people talk about the volume of a shape, they're probably talking more about how it looks. When people talk about the mass of a shape, they're talking more about its weight. It's like two sides of the same coin. The word 'massive' means huge.
Size is a powerful tool in your art box, the key options being large and small. It's important to remember that...
...size is relative.
The shapes in your work don't have to be extremely different sizes. People will see them in relation to each other. So, a little circle is huge compared to the dot next to it.
Proportion is what controls your shapes. It determines how tall, short, thin, or fat they are. Proportion is extremely important in realism. When drawing realistically you don't have to copy every detail you see - you're not a meat camera. But you do need to accurately see and copy the proportions, using your eye as a ruler and protractor, to measure lengths and angles.
Space is the area around your shapes. Positive space is made up of objects, and negative space is the empty area behind your objects.
Colour is pretty,
Boulders Near Bear Cliff, by Charles Courtney Curran
except sometimes when it's ugly:
village church, by Chaim Soutine
(would you like some mustard with your painting today?)
In America, we spell it "color". The three elements of colour are:
hue - a colour's position on a colour wheel
chroma - it's intensity or saturation
value/tone (odtien) - it's range from light to dark. Value is also present when colour isn't.
Colours can be flashy and help make artworks more memorable, because they tap into our emotions and memories.
Texture is the look and feel of a material. It can be rough or smooth, sharp or soft, flat or glossy, etc.
Contrast just means difference. People naturally see differences. If you open Photoshop or similar computer programs, you may think contrast is just about light and dark, but it's not. To quote illustrator Tristan Elwell:
"There are many different kinds of contrast, not just contrast of value. There's also contrast of hue, chroma, size, direction, texture, detail, etc. You use as many of these as possible (depending on your medium) to make the viewer look where you want them to. You can also use some to balance or compensate for others."
Molly Bang explains in her book, The Principles of Composition, that contrast is what enables us to see. Without contrast, everything would be an even fog of grey.
Perspective is a trick of design to create the illusion of 3D space on a 2D surface, based on a horizon line and vanishing points. There are different kinds of perspective, varying in complexity. To draw realistically you must learn to see and draw in perspective.
symmetry & balance
Symmetry is the division of space into smaller identical spaces and shapes. There are different kinds of symmetry. Most often, it functions like a mirror. There is also radial symmetry. Balance also divides pictorial space, but the resulting shapes are not identical. Balance suggests symmetry without being exact. It's more relaxed.
repetition & rhythm
Repetition is when you repeat the same or similar line or shape several times in an artwork. Rhythm has to do more with where you put these repeated shapes and what line or shape is made when you look at them all together.