Hi Nikki
NSI is one of the biggest professional brands, with CND (Creative Nail Design) being the largest global pro brand. BRiSA is their gel system and Shellac is their gel polish - applys like polish, wears like gel.
Here's a quick system summary that should help when looking at brands and their uses:
Liquid & Powder (acrylic) strong enhancement product, ideal for clients who want ultimate strength and design from an extension.
Hard Gel (has to be filed off) - eg NSI gel or BRiSA also a good enhancement product for strength & length, preffered by some techs for ease of application
Soft Gel (can be soaked off) - eg Biosculpture, good for those who can maintain long natural nails, but preffer the look and feel of having an overlay
Gel Polish eg Shellac - ideal for clients that really only want polish, but hate how fast it wears. These will hold and wear much longer without chipping or flaking like normal nail polish..
Most of the brands you are looking at can offer samples, however they may or will ask for proof of qualification depending on what system you are requesting - specifically for the pro products (gel upwards on the list...)
I would recommend looking through the websites of each brand you are thinking/heard about and contact them to see how they fit with your intentions...
Another good recommend is to get Doug Schoons product chemistry & natural nail anatomy book - it will give you a much greater insight into products ...