Please explain to me what is the different between acrylic, gel, silk, and crystal nails. I often hear manicurist say that crystal nails are better than the rest of the products. It seems like acrylic would damage my nails. Is it true.?

Acrylic is a liquid & powder mixed, applied with a brush and will harden (cure) with no lamps in 2 minutes or less. 

Gel is, in basic terms, pre mixed acrylic that almost always needs to be cured under a UV lamp, Acrylic usually has an odor while gels are odorless, but both have vapors and need to be ventilated. 

Silk (interchangeable with fiberglass and sometimes linen) is applied with resin (glue) and sometimes glue & acrylic powder. 

Crystal nails is any enhancement done with CLEAR acrylic, gel or resin. 

Acrylics (or any enhancement for that matter) do not damage nails, poorly trained technicians damage nails.