The zest contains essential oils which have a strong scent and flavor of lemon. Use a microplane to scrape off the yellow zest, leaving the white pith behind. As Kristen said- the pith is extremely bitter. The juice is from the interior of the lemon. It is very tart, and is usually sweetened when used in baked goods. Zest can elevate a simple sugar cookie recipe or transform a cake. Lemon zest is also great on pasta with a little parmesan and olive oil.

The zest contains the oils...which adds a depth of flavor without the tartness and "liquidity" of juice.

Lemon zest is just the yellow skin, grated right off the lemon (without the bitter white pith). And lemon juice is the liquid squeezed from inside. The former is more fragrant and less tart.