Oracal 631 is the most popular choice of vinyl. It is very versatile and compatible with all vinyl cutting machines. This product is commonly referred to as "indoor vinyl". It certainly is well suited to indoor applications but is not limited to them.
The water based removable adhesive is what makes it a great choice for indoor applications. It will not damage walls when removed assuming the walls are in sound condition. This is also the same feature that leads people to believe that it is not suitable for outdoor applications. Oracal 631 can get wet, it just is not a great choice in areas that it will be constantly exposed to water.
Oracal 631 Specs
Oracal 651 is a performance grade vinyl well suited for indoor and outdoor applications. It was originally designed for fleet vehicle graphics making it a perfect choice for cars.
The solvent based permanent adhesive makes it a great choice for wet area applications such as boats or other outdoor places. It can still be used indoors on walls but has a much higher potential to cause damage when removed.
Oracal 651 Specs
This chart will help you choose the correct adhesive vinyl by your application.