Citrus fruits come from flowering trees and shrubs that are members of the Citrus genus. Citrus fruits are notable for their aroma, juiciness and acidity. Apples are not citrus fruits. They are pomaceous fruits and are members of the rose family.
Pomaceous fruits are accessory fruits, meaning that the fruit itself is not wholly derived from the flower's ovary. These fruits are typically fleshy and have a sweet flavor. The apple is the best-known example, but other examples include pears and loquats.
Lemons (Citrus liman), limes (Citrus aurantifalia), oranges (Citrus sinensis), grapefruits (Citrus paradisi) and tangerines (Citrus reticulata) are all members of the Citrus genus. They vary in flavor from sweet to sour.
Citrus plants typically grow as shrubs or trees and can reach up to 49 feet in height. They have dark green leaves and often have sharp spines along the branches. The flowers are generally small, white and strongly aromatic. Citrus plants have a tendency to hybridize, and there are many species, crossbreeds and cultivars.