The strength of a band is determined by its thickness and is colour coded for easy identification. Generally, lighter colours are easier and darker colours are harder. Depending on what you are using them for will determine what strength band you need.
If you are using your resistance band for strength exercises, a darker band will make the exercise harder than a lighter band. Stretching is similar; a lighter band will need to be pulled much harder to get them same stretch a harder band may provide. This all depends on your strength or flexibility level – a beginner will be better starting with a lighter band and advancing to a darker band.
On the contrary, if you are using a band to assist you (e.g a strength band for chin-ups) a darker (thicker) band will provide more assistance than a lighter band, making the exercise easier.
There are many different types of resistance bands. Plain resistance bands can be used for almost anything, loop around your feet or a pole to use two hands; tie around a pole for single arm use. Resista Loops are excellent for rehab exercises or stretching and are great for aiding limb alignment.
Strength bands are great for increasing or decreasing difficulty of exercises. Loop around the ends of a barbell to change the dynamics of a bench press or make body weight exercises easier.