|19th century illustration|
It is a healthy food (but rather fatty - it is one of the fattiest cooking oils in use, even more than palm, canola and other vegetable oils). Olive oil is the juice made by crushing olives. It is considered to be healthier alternative to sunflower oil.
In Roman literature, the olive branch is used as a symbol of peace: "extending the olive branch". In Christianity, too, it is seen as a symbol of peace, because according to the Bible, a dove brought an olive branch to Noah to show that the flood was over. Lots of important writers and illustrators use olive branches around text to give it a greater artistic credit (a similar effect is achieved with wheat).
The raw olives are very bitter. They are "cured" mostly by bacterial fermentation. Later they may be alternately washed with water and packed with salt. The procedures vary a lot, and different varieties of olives get different treatment. The bitter chemical oleuropein, a phenol compound, has to be broken down or removed. This is what the curing process does. Even so, olives still often do have a rather bitter taste.