Having dogs and cats get along can be important if you happen to have one and want to add the other. The first step in evaluating how much effort it will be to make this a safe combination is something called a cat test. A cat test evaluates a dog for how safe he or she is around cats and hence determines what the dog training effort is going to be to make it work. Generally speaking, most dogs will be able to live with cats but very high-prey drive dogs will have a harder time leaving cats alone and changing this can take a while.
The following link point to an excellent write-up by a trainer who runs a Greyhound rescue. Her article was written for Greyhounds, which tend to have higher prey drive, esp. if they were racing previously, but it is 100% applicable to cat testing all dogs and a great source of information for anyone who wants to know HOW this is done.
Performing an actual cat test should however never be done without hiring a professional dog behaviorist, as not seeing the signals the dogs and cats exchange, can have fatal consequences.
On Cat Testing: Cat Testing and Introductions