A green vegetable or Karpas (in Hebrew) symbolizes the spring time. It also symbolizes the bitter times the Jews had to endure in slavery.
It is believed the stem of the lettuce is bitter and as it grows up from the ground, the leafy part begins to taste less bitter.
The most common green vegetable which is used is lettuce. Some people use celery. The Maror and Charoset is placed inside the lettuce leaf and eaten.
The Karpas is also used to dip into the salt water at the beginning of the seder.