What are meristems?

What are meristems? How does the primary growth differ from that of secondary growth?

The growing regions of plants are called meristems.
Primary growth refers to the elongation (growth in length) of stems and roots. All plants exhibit primary growth. Root apical meristem and shoot apical meristem are responsible for the primary growth of the plants and principally contribute to the elongation of the plants along their axis.
The growth that make plants thicker (growth in diameter) is called secondary growth. For example, in dicotyledonous plants and gymnosperms, the lateral meristems, vascular cambium and cork cambium appear later in life. These are the meristems.