Tahini is ground sesame seeds, generally without the hulls. Sesame butter is also ground sesame seeds, but generally includes the hulls. It has a nutty, slightly sour taste. It is a common ingredient in many Middle Eastern, Indian, and Mediterranean foods, such as hummus, baba ganoush, halvah, and many sauces and dressings.
In Arabic, it's known as tahina, while in Hebrew, it's known as t'xina (pronounced "t'ch'ina").
There are two different kinds of tahini generally available—'hulled' and 'unhulled'. The hulled version is made from seeds which have had their outer husks removed. This makes it less bitter and lighter in colour, but also less nutritious because a lot of the vitamins and protein are contained in the outer hull. In some places you can also get tahini sweetened with sugar or honey—needless to say, this version is not suitable for use in savoury dishes.