First, a little history...
Back before computers, when type was set by hand, great care was taken to make a page of type appear "beautiful." The typesetter would have the luxury of analyzing each line of type to make sure it didn't create unwanted text disturbances like rivers, widows, orphans, or hyphenation problems. If an aesthetic problem was identified, it, along with any other lines of type affected by the change, could be quickly remedied.
Now, fast-forward to the computer age. Even though computers have made the job of setting type much more efficient, the software has always lacked the ability to analyze multiple lines of text in order to achieve the best aesthetic typographic result.
Enter Adobe's InDesign, and the introduction of the Adobe's Paragraph Composer, which has the capacity to reduce the amount of time spent on composition, and increase the consistency of hyphenation and overall letter and word spacing.
Adobe's Paragraph Composer can consider multiple lines of text, eliminating widows, orphans and text rivers, and improving the overall quality of the body of text as you type, allowing you to approach page layout from an artistic point of view.
Preferences for Adobe InDesign's composition engine are defined by selecting the Adobe Paragraph Composer or Adobe Single-line Composer from the InDesign Paragraph palette menu.
Choosing whether to use the Paragraph or Single-line Composer depends on what type of work you are doing. If you are working with a small amount of text, such as a headline or caption, the Single-line Composer will allow more user control. The Paragraph Composer is best suited for larger bodies of text because it was designed to consider multiple lines of text at one time, and will provide the highest-quality aesthetic results with very little hassle.