Play between cats and dogs can be fun for both species. However, cats that have not been socialized with dogs from an early age will almost always behave defensively. These cats will either flee or demonstrate an aggressive display when they encounter a dog.
Even if they seem to be playing well together, dogs and cats can have quite different play styles. These differences can lead to problems even if the intentions are good on either side. Dogs can enter the situation too eagerly for your cat to feel comfortable. Supervise play closely, so that you can interrupt and redirect your dog if they become overly excited. Give your cat a safe zone and a clear path to retreat if they do not feel comfortable. Injuries from a cat to a dog can be severe and involve scratches and/or bites to the eyes, face, and body. Injuries from a dog to a cat can be severe or even lethal due to the size and strength difference of the dog.
Always introduce dogs on a leash to reduce stress for your cat and avoid any chasing. Cautious investigation, tail wagging, and backing off when your cat gives defensive signals are all good signs from a dog. Bad signs are instant attempts to chase, out-of-control straining at the leash, whining, barking, and general agitation.
There is a range of temperaments in cats as well as dogs. This will influence the success of any dog-cat play. In general, relaxed, laid back cats and kittens are the best candidates to accept a dog. They are also less likely to flee and trigger chasing. This will allow a social, rather than a predator-prey, relationship to develop. Dogs with low prey drive are more suited to play with cats.