Learning terms associated with art and art objects is a valuable, though practically endless proposition. You can think of the terms as coming in two categories: those you already know and those you don't. I think it is best to start by reminding ourselves of those we already know. These include, but are not limited to the following:
First, think of the formal terms that help us make our way around the composition. These will include: point, line, shape, volume, space, texture, with the various aspects of motion, direction or other energies these may suggest or create. Also, we learn to look for shapes and volumes to see larger and smaller patterns: circles, squares and rectangles, triangles, and the e-dimensional spheres, cubes, pyramids.
Also first , we make sense of whatever light there is, its qualities and source(s), and how the light relates to shadows and darks.
When it comes to color, we may need to remember that hue is the name of the color, that value or tone refers to the lightness or darkness of the color, and that saturation or intensity refers to its brightness or dullness.
Always, though, we ask, whenever we remark on something in a work of art: What is it and what is it doing? What are they doing together, whatever they are? In this, we learn to look at works of art in terms of both the larger composition itself and the various patterns and arrangements of repeated elements or qualities we find.
Repetition is one of the most common devices in art of all sorts, meaning essentially that just identifying repetition as such is dull. Things that repeat may call attention to themselves, so that they become more noticeable and, hence, more expressive as a result. Themes and motifs in works of art become what they are because they repeat. The various ways elements repeat create various sorts of identifiable patterns..
Parallelism refers to how two or more elements of a work relate to each other by some significant means, generally because they are similarly placed or sized, so that one echoes the other. (Incidentally, since parallelism generally provides clarity of relation to the parts at hand, the best advice I can give to struggling writers is to nail sentence parallelism.)
Antithesis & contrast organize the relationship of parts - usually just two - in opposition; we tend to locate meaning by way of contrasts: If this, then not that. Contrasts are found in formal elements - rectilinear and curvilinear lines, light and dark - and in thematic elements - creation and destruction, life death.
Rhythm is a particular form of repetition based on an on/off pattern. Like other structural elements, rhythm may be found in any sort of art, in sound and vision. In the visual arts rhythms are frequently created out of repeated lines or shapes that parallel each other. Rhythms tend to be highly self-conscious, or at least highly structured, so that even things that do not themselves move can appear to move. Visual rhythms are often appealing and may suggest the dance.
Symmetry refers to the relationship of parts based on similarity and balance. Like other elements of structure and cohesiveness, symmetry creates echoes among different parts of a text, and is found in virtually all the arts.
Juxtaposition refers to how close or contiguous the various elements of a piece are to one another. Ordinarily, of course, we take no notice because of its common quotidian character - something is always next to something else. When we do notice related parts of a text, we may find special relevance in their being juxtaposed.
As we learn to name the parts of a work of art and to identify how those parts relate to each other and to the whole, we may become increasingly aware of the myriads of things in the world we live in which we can identify by name in any given work of art. Just starting to account for almost any visual portrayal can start taxing our powers of language. How many parts are there in a work of art? Too many to count, if you start putting everything together. But that practice can take us to a place in our experience of the work at which the piece starts to (seems to) interact.
For all of the above, we already know most of the terms mentioned. All we need to do is gain some practice applying them to works of art. Probably the best way to do this is to start talking - to yourself and to others - about what you find, and writing down - in sentences as well as paragraphs - what you observe.
Artlex May be the biggest and best of the online lexicons
Architecture Dictionary is a fine, well-illustrated site
Architecture dictionary Some terms are illustrated
Sculpture dictionary Looks like a useful site
Medieval Art and Architecture A very good site