Centrioles (a.k.a. Basal bodies) are found in many* cells that have a nucleus. They were discovered when advances in microscopes made it possible to look closely at the inner structure of living cells.

They feature a central cylinder or barrel-like structure, about 450 nm high and 250 nm in diameter, with various 'appendages' attached in a nine-fold symmetrical pattern. It's known that the structure features more than 100 different proteins.

The centriole is a magnificent molecular assembly of several giga-daltons, one of the largest of the eukaryotic cell, and whose atomic structure remains unsolved to date. However, numerous electron microscopy, cryo-tomography, and super-resolution studies now make it possible to establish a global architectural view of it with its different sub-regions. These analyses broaden our understanding by providing additional informations to cell biology and structural biology approaches. In this review, we describe current knowledge on the overall organization of the centriole. We will highlight each sub-structural element, their differences between species and their putative protein composition. We will conclude on the current limitations that still take us away from a complete atomic view of the centriole architecture.

Source : Current Opinion in Structural Biology, Volume 66, Pages 58-65(with many detailed illustrations)

As the paper cited above points out, after more than a century of research, much of the fine detail of the structure of centrioles is still unknown. In addition, the function so-called 'distal' and 'subdistal' substructure is currently unclear.

It's known that they are involved in the process of cell-division, and are essential for the formation of cells which have centrosomes, cilia, and Flagellaplugin-autotooltip__plain plugin-autotooltip_bigFlagella

A flagellum is a lash-like movable appendage - often used as a means of propulsion - which is attached to the cell body of many bacteria and some eukaryotic cells. There are some notable examples in plants (e.g. fern spores) and even mammals (e.g. sperm cells)..

*Note : Flowering plants, conifers and fungi don't have centrioles.