It's true that a sequin is a form of currency in Turkey and Italy, but who wants to hear about that? What you really want to hear is...
A sequin can also be a small, shiny, ornamental disk, often sewn on cloth, mainly found on pieces of women’s clothing. These are sometimes also called spangles.
Usually small and round, with 'bends' in them so they depress in the middle for easier sewing and also so they reflect light better, making the article of clothing more noticeable and bright.
Sequins can be used sparingly or they can cover the complete object. Uses range from a small amount on a hem or border, to completely enveloping the whole article of clothing or accessory. There are sequin purses, gowns, dresses, shirts, pants, and hats; basically anything that a thread could pass through could have a sequin on it.
Sequins are applied by sewing them on to the desired object. You take a small needle and thread, put the needle through the hole in the middle of the sequin and then pass the needle through and back out of the article. Place another sequin on the thread and repeat. Be sure to keep stitches even and steady so your pattern doesn't look sloppy or careless. It takes practice.
Sequin takes on another definition that has absolutely nothing to do with sewing or apparel. Sequin is also the name of a scientific software tool developed by the N.C.B.I. for submitting and updating entries to the GenBank databases. Basically it's for submitting huge readouts of DNA to a databank. It is capable of handling simple submissions which contain a single short mRNA sequence, and complex submissions containing long sequences, multiple segmented sets of DNA, or phylogenetic and population studies.