Most times, landscapes and yards will have a definite defining point between the backyard and the front by means of gates, fences, or plantings. However, there are times when it's desirable to have the entire landscaping open with the home sitting in the middle of it. And while the front and back may have different purposes with different plants and elements, it may be necessary to create not so obvious transitions on the side yard.
Usually, plants are the best elements to work with for creating transitions as they can be shaded down or up to meet the next area. See planting ideas for more on choosing, grouping, and working with landscaping plants.
Along with transitioning with plants of different shades, sizes, and textures of green, a single repeated hardscape or softscape element among the side transition will help keep continuity and unity.