There are three major categories: Strand by strand extensions, weft extensions, and clip-on extensions.
With this extension technique small strands of extension hair are attached one by one to small sections of your own hair by either weaving in, gluing, heat fusing, clamping with metal tubes or using waxes and polymers.
Glue/Bond based attachments
Do-it-yourself Hair Extensions
NON Glue/Bond based attachments