Halacha requires eating Charoset at the Seder on Pesach, and two reasons have been given for this requirement. Firstly, the Charoset symbolizes the cement with which the Egyptians forced Benei Yisrael to work in preparing bricks during the period of bondage. The Charoset is therefore made in a consistency resembling that of cement, generally from crushed fruits; Sepharadim generally make the Charoset from crushed dates. It is also customary to add sweet spices (some add cinnamon), walnuts and wine, in order for the Charoset to resemble cement.
The second reason given for the Charoset is the tradition that during the period of Egyptian bondage, the women of Benei Yisrael would deliver their infants in the apple orchards in an effort to escape the Egyptian authorities who were ordered to kill the newborn males. This is mentioned in a Pasuk in Shir Ha'shirim (8:5): "I have awakened you underneath the apple tree – there did your mother deliver you…" The ground miraculously opened and took in the newborn infants, who were fed and cared for by angels underground and then returned to their parents upon reaching maturity. We commemorate this extraordinary miracle by eating Charoset, which consists of apples or other ingredients resembling apples.
When eating Marror at the Seder, we first dip it into the Charoset. This is generally explained as intended to diminish somewhat the pungent taste of the Marror. Additionally, perhaps, we might explain this custom on the basis of the aforementioned miracle that God performed for Benei Yisrael in caring for the newborn infants. By dipping the Marror in the Charoset, we commemorate the fact that there was "sweetness" even amidst the bitter suffering of oppression, in that God miraculously cared for Benei Yisrael's children who were eventually freed from Egypt.
One should not scoop the Charoset onto the Marror, but rather dip the Marror into the Charoset and then shake the Charoset off the Marror, so that only a small amount remains to diminish somewhat the sharp taste of the Marror.
It is permissible to eat Charoset on Erev Pesach as well as throughout the holiday.
Summary: Halacha requires dipping the Marror into Charoset at the Pesach Seder; after dipping, one should then shake the Charoset off the Marror so that only a small amount remains. One may eat Charoset on Erev Pesach and throughout Pesach. Charoset is generally prepared with crushed fruits, sweet spices, walnuts, wine and apples.
