These straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells (prokaryotes don't have them) and perform a number of functions.
Microtubules form part of the cytoskeleton that gives structure and shape to a cell, serve as conveyor belts moving other organelles through the cytoplasm, are the major components of cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis). Microtubules can function individually or join with other proteins to create larger structures (e.g. cilia). These filaments are composed of linear polymers of tubulin, which are globular proteins, and can increase or decease in length by adding or removing tubulin proteins.
Information provided by: http://micro.magnet.fsu.edu