Congratulations, you are getting married and planning one of the most important days of your life.  You have the date and the venue and you begin to look for someone to marry you and before you know it you are lost in trying to discern the differences of just who can marry you in Massachusetts!  So here it is in an easy to understand breakdown for you:

• Justice of the Peace: A JP is an appointed position through the Governors Counsel in Massachusetts for a period of 7 years and fees for services are regulated by the State.  Depending upon the JP you speak to and your wedding location they start with a regulated "flat fee" to this they can add additional fees for pre-marital interviews, planning sessions, rehearsals, travel and even customization of a ceremony, (i.e. there are JP's that actually may charge for the adding of a ritual).  By law they are required to put in writing all their fees that you will be assessed.  Usually (but not always) the ceremony is a standard civil ceremony which is then tailored to your specific information.  Some JP's are also members of the clergy.  If this is the case they may offer other services but be clear about the capacity in which they are solemnizing your wedding license.   If they are signing as a "JP" then their fees need to be reflective of this.
• Member's of the Clergy affiliated with a religious house of worship (temple, church, sanga mosque, etc.): Usually there is a recommended donation to the place of worship.  Ceremonies are designed to honor specific traditions, tenets and dogma.  They are more ritualized and adhere to a particular order of ceremony.  While there is definitely customization, the amount of this depends upon the individual clergy.
• Members of Clergy and Celebrants who are not affiliated with a specific house of worship:  Ministers, Non-denominational Ministers, Interfaith Ministers, Unitarian Universalist Ministers, Humanistic Ministers and Celebrants all fall into this category.  There are a wide array of services from a simple basic ceremony using a "template" to a fully customized ceremony in which each part of the ceremony is designed for you.  Each of us has our own style and willingness to spend time with you and allow you to be be part of the creative process.  Make sure you understand the process and willingness to work with you until the ceremony is what you want.   Fee's can generally range from $300 - $1200 and often reflect the time an officiant is willing to commit to the design process.  Read their testimonials and ask for references if no testimonials are available.  Again, make sure all agreements are in writing so you understand what the total cost will be.
• Anyone you designate:  A friend or family member who is of legal age can solemnize a wedding in Massachusetts as long as they apply for a 1 day designation.    If you decide to go this route...please read my blog entry, "Should a Friend or Family Member Officiate at your Wedding" from May 22, 2009

I hope this helps.  Now that you understand the differences you can intelligently put together questions that are important to you as you begin to search for your wedding officiant.  There are no wrong choices, the most important aspect is that you feel comfortable and connected with the person that will be marrying you.  If you haven't already done so feel free to read my article on selecting a wedding officiant.