An arborist is trained in tree care and has the proper training and equipment to work in and with trees. However, there is a big difference between your average arborist and an ISA certified arborist.
Arborists who have been ISA certified have a minimum of three years of field work in the tree industry, completed a course of study, and passed a comprehensive exam to attain certification. In order to stay ISA certified, the arborist must always be involved in continuing education as the certification requires a certain number of CEU's (Continuing Education Credits) be obtained each year. This insures the homeowner that this person is trained on the latest science and technology with regard to tree care.