Pretty much all vinyl can be used outdoors, but there are types that are better suited for outdoor applications.  Vinyl is typically used for sign making and most signs are outside, so most vinyl is designed for outdoor durability.  The best vinyl for your application will be determined by a few different factors.

Generally speaking a calendared film, such as Oracal 651 will work fine for most applications.  Cast films do hold up a bit longer but usually the additional cost is much higher and makes it not worth it.  The difference between calendared and cast films is not really visually apparent so most of the time either one will work. 

Most vinyl that is well suited for outdoor use also has a solvent based permanent adhesive.  This gives the adhesive a good bite and will stay stuck down for many years.  This doesn’t mean the vinyl is not removable, it just means it is a little more difficult to remove.  Any vinyl can be pretty easily removed by warming it up with a hairdryer.  Any adhesive that gets left behind can be cleaned up with a product like goof off. 

Products like Oracal 651 are specifically designed for fleet vehicle graphics use.  This gives the 651 product two very good features. The first is great durability to extreme elements.  The second is it has a very affordable price tag.  As I mentioned before there are other films that are slightly more durable, but the cost is two to three times more expensive.