Spirit guides are just what they sound like.  They are spirits who help "or guide" you from the other side.  Every person has a spiritual guide for life "the main guide" if you will.  This guide is also called a guardian angel, and in most cases is a High being of light. 

You can also have lesser guides throughout your life.  We believe your guides are chosen by you before your incarnation on earth.  They are not with your through your entire life time, but show up to help you in particular areas of your life as the lessons you have chosen begin to take place in your life.

Many lower guides will come into play when you are attempting to contact your higher guide, in order to help you establish a connection.  Once the connection is made and strengthened over time, the lower guide has done her job and is no longer necessary. Your higher guide or guardian angel exists on a plane with a much higher vibrational rate, and it is not easy for them to come through to you here on earth until you have "raised your own vibration."

Deceased relatives can act as guides for you as well.