Bonder is a term that refers to a Monk who specializes in protecting the party by placing Life Bond and/or Life Barrier on all or some of its members. In PvP, to help against spike teams or other cases where Life Bond will not help, bonders sometimes also carry Vital Blessing.
Bonders generally run high Protection Prayers and Divine Favor to strengthen their maintained enchantments and provide energy to maintain them through Blessed Signet. When using Life Bond, bonders will usually use Balthazar's Spirit on themselves to provide additional energy. Mantra of Inscriptions can help provide more energy, but when using Life Bond and Balthazar's Spirit, it is not strictly necessary. Maintaining Essence Bond on a tank can also return some energy for the bonder, but if you are bonding more than one person the degeneration can outweigh the energy gain.
Bonders must spend much of their time and energy maintaining their enchantments and so are not as flexible or responsive to enemy actions as an active prot monk would be. Energy denial such as Nature's Renewal or disruption of Blessed Signet can force a bonder to drop most of their maintained enchantments. Since Life Bond and Life Barrier can only be maintained on other allies, the bonder is often the weakest target on a team. In PvE this is not much of a problem, but in PvP, some teams bring an extra copy of Life Bond on another character just to use on the bonder.
Since the addition of Nightfall professions and skills, Paragons have taken up the role of the bonder in high-end PvP matches. Angelic Bond behaves similarly to Life Bond, but avoids its downfalls such as energy degeneration, Nature's Renewal and enchantment removal. However, Angelic Bond does not reduce damage to the Paragon like Life Bond, so the damage needs to be reduced with skills like Shield of Absorption and healing skills.