There are many names for this material. The most common ones being acrylic and plastic. They are also sometimes referred to as Perspex or PVC. As an end user with no other specific requirements other than for display purposes, the above names all refer to more or less the same material. More specialised plastics that either has a higher melting point or higher rigidity such as traffolite and polycarbonate will be discussed in a separate article.
The most common thickness for general signage usages are 3mm and 5mm, and the range of easily accessible thickness is from 2mm to 10mm. The above image is an example of a 10mm thick transparent acrylic piece, with engraving and blank filled paint. Naturally, the price increases accordingly to the thickness except for 2mm acrylics, which will be more pricey than 3mm acrylics due to it being slightly more rare.
The cheapest acrylic signage option would be a 3mm thick acrylic with a printed vinyl sticker laminated onto it, be it direct or reversed. This refers to the side of which the sticker is laminated onto the acrylic. In general, most would opt for the reversed option, as the printing would be protected by the layer of acrylic and as a result last for a longer time.
These acrylic products are generally rather light in weight, and installation for sizes up to A3 size can be done with 3M foam tape. Larger sizes would require screws for safety reasons.