Composition characterizes the objective visual traits of the art. Look at how lines, color and textures are used. These give an artwork its style and form. For example, if you were to compare Picasso's "Bottle of Rum" with Robert Nugent's "Wine Bottles," the first thing that might strike you is the difference in color, line and perspective. Rather than discussing color, line, composition and perspective, it's simpler to identify each painting as having different styles. Picasso's painting style is non-representational, which means it's not obvious that his painting is of a bottle, whereas Nugent's painting obviously shows several bottles of wine. Picasso's painting is linear (focused on line) while Nugent's is painterly (focused on highlights and shadows).
Content is a little more subjective; it has to do with what the painting or sculpture is about and what that means to the artist as well as to the audience. The content can also provide clues as to why the artwork was made. For example, Michelangelo was commissioned to sculpt the tomb of Pope Julius II. He chose to depict Moses at its center, and in this sculpture Moses, curiously, has horns. This leaves a message for the viewer to walk away with. Michelangelo was a serious Catholic and anti-Semite. The content of a work of art gives the viewer an extra sense of context.
The medium describes the material from which the art was made. For example, there are paintings, drawings, sculptures, architecture, photographs and graphic arts. Within each medium are other specific media. For example, Leonardo da Vinci painted "The Last Supper" using oil paints on a wall. This matters because he was attempting to paint in the same style as frescos, but frescos involve painting with the wall plaster itself. Da Vinci didn't do this, so there is hardly anything of his original left despite restorations. The details behind a medium used in artwork add to the story of the artwork, and in some sense the medium itself is the message or at least defines its expressive potential.
The period in which a piece of art is made inevitably provides the historical context for the artwork and guides interpretation. It also serves as a way of categorizing the art. For example, "Impressionism" refers to a specific period of painting by French artists, starting in the 1870s. Artists such as Monet and Degas and Renoir delved into a spontaneous, open-air painting style that emphasized their impression of light in a scene. The impressionists also took on more scenes of leisure that portray life in a positive way. What you may find is that groups of artworks that have similar composition, content and media are from the same period.