Nerve fibers can be classified based on their structure and distribution. There are two main types - myelinated and unmyelinated fibers. Nerve fibers also include somatic and autonomic fibers. Somatic fibers innervate skeletal muscles and transmit acetylcholine, causing muscle excitation. Autonomic fibers innervate smooth, cardiac, and glandular muscles and can cause excitation or inhibition. Important properties of nerve fibers include excitability, conductivity, unfatigability, refractory periods, all-or-none response, summation, and accommodation.