CR=Average Party Level (APL) usually yields a Medium encounter. (Recall the encounter descriptors in "Combat Encounter Difficulty" on DMG p.82.) The exceptions are at levels 5-7, where the "CR-appropriate" monster actually yields an Easy encounter. CR=APL-1 usually yields an Easy encounter. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.