Catholics believe that when the priest prays over the bread at Mass it becomes the body of Christ, totally and completely; this is not just a symbol of Jesus but an actual, real presence. Jesus becomes fully present there in the bread. This is why the Eucharist is such a holy thing; it is not just a piece of bread anymore; it is Jesus Himself! Catholics know this is true because Jesus told them in John's Gospel that 'this is my body', 'it is real flesh'. He didn't say 'this is a symbol of my body, this is sort of my flesh'. The Catholic Church is as clear about what the bread is as Christ Himself was.

'The Blessed Sacrament' is the name of this blessed bread, the body of Jesus, and it is often kept aside for people to pray in front of outside of Mass. This is called 'Adoration of the Blessed Sacrament'; time spent with Jesus in the bread that has become His body through the prayers said over it at Mass.

See:
http://www.youtube.com/watch?v=-DLzz0DEpAk&feature=related