A pattern is, like rhythm, constituted of similar elements. However, unlike in rhythm, the eye is allowed to roam freely over the whole surface of a pattern. It is this lack of a strong direction which lends the pattern its distinguishing trademark: of having a static feel.
Pattern is about area, where rhythm is about direction.
Pattern is most effective when it fills the frame, because when bounds to repeated elements are absent, the viewer's mind will assume that the pattern is continuous beyond the image frame. But the caveat to this is scale i.e. the ratio of the individual element's area to the image area; and number number of elements.
Take for example, grains of rice. Filling the frame with a close-up of three grains of rice does not create a pattern. Conversely, an image of a surface covered by ten thousand grains - where each grain is perceptually indistinguishable - creates a texture instead. Thus scale is the main differentiator between pattern and texture.
To achieve a pattern, take an image of a group of similar elements such that the whole group just fills the frame, and then take images at successive increments closing in until just a few elements occupy the frame. The feel of the pattern will be felt strongest somewhere in that sequence.
Regular patterns comprise geometrically regular layouts. Visual interest is highly dependent upon the nature of the objects: a grid of oval cameos or coloured glass marbles with inclusions would yield and internally dynamic pattern than one of black rubber washers.
Irregular patterns arise out of unordered layouts. The closer the elements are grouped together, the less the obvious the irregularity (consider the grains of rice example above). Close groupings use the Gestalt Principle of Proximity, where items close together are perceived as belonging together.
Breaking the pattern plays on the directionless, meandering nature of patterns which makes them well suited as backgrounds. A contrasting element placed on/in the pattern is an immediate attention-grabber. Think of a still-husked grain rice, on a pattern of unhusked grains.