Velvet is a woven fabric with a short dense pile which gives it a luxurious soft feel. Velvets have a good amount of drape. Velvet does not have a natural stretch but can have elastane woven into the fabric to give it some stretch whereas velour is a plush knit fabric that has a natural stretch.
Sequin fabrics normally have individual sequins sewn onto a mesh base. These sequins are normally made out of plastic and therefore aren't biodegradable.