According to the definition of "mortal sin" it is. As far as "swearing" it depends on what you consider swearing.Most consider taking God's name in vain as swearing. "Thou shalt not lie" and "Thou shalt not take the name of the Lord thy God in vain" are two of the Ten Commandments so according to the Catholic and regular Christian beliefs, lying and swearing are mortal sins.
Apple_fan is correct. In the Catholic Faith a mortal sin is one that cannot be washed out even with the blood of Christ. According to Catholics once you have committed a mortal sin you will end up in Hell. Murder is a mortal sin.
A venial sin is one in which Christ's blood makes you whole as long as you repent and are truly sorry for what you did. Going to Confession wouldn't hurt either. BTW I am not a Catholic but was raised in a Catholic family with all that entails.