Animals disperse seeds in two main ways - 1.) by eating the seeds and then shedding them in their droppings and 2.) physically carrying the seeds to other locations.
Seeds which are ingested need to be protected from damage by the animal's teeth and also from the harsh conditions of the digestive system. For this reason, seeds are usually very hard so that they can not be easily crushed between the teeth or so small that they can be swallowed whole (for example, raspberry seeds). Seeds also have a tough outer seed coat which is not penetrated by the stomach acids. Thus viable seeds can be passed through the digestive tract.
Some seeds are large and not particularly tasty (for example, a peach pit). This prevents the seed from being eaten along with the fruit in the first place. An animal can carry a fruit like this to a new location to eat it, and then just eat around or spit the pit out.
The cashew is a particularly interesting plant. The cashew produces a 'cashew apple' and the cashew itself is attached to the bottom of this apple. The apple is very tasty and is food for humans and other animals. However, the cashew is the actual seed, and is encased in a hard shell which contains a chemical which is irritating to the skin. In this way, animals are enticed to collect and eat the apples, dispersing the seeds in the process, but are less likely to try to break open the seed itself.