A long throw lens is a special type of lens that allows a projector to be placed farther back without stretching out the screen as far as a normal lens would. Long throw lenses are usually external lenses that are mounted on in front of the normal lens, like a pair of glasses. Some projectors will come stock with a long throw lens, but this is very rare and are often from name brands you cannot trust. Typically you can find a long throw lens from any where from $1000 for a very cheap one to all the way up to $5000 for a high quality lens for very long distances.
A long throw lens is typically used in a larger facility like a church. A church for example would not want a bulky projector in the middle of the church, but would rather have it placed in the back of the church where it is far out of site. Also University lecture halls, reception halls, and convention centers are often a common place for long throw lens due to the buildings large size. Another common place you might see a long throw lens is a school auditorium. If you have ever looked at the very back top corner of a school auditorium there is usually a sound and video box where a projector is placed. Now think of how big that projectors screen would be if it did not use a long throw lens, so in most cases like this a long throw lens a must have.
One last reminder when planning your budget and seeing how much money you want to spend. Consider whether or not you want a long throw lens.