Ok, so these two words are used pretty interchangeably in sci-fi material.
That don't make it right. There's a difference.
If I had to cut it quick and dirty:
Sentient = ability to feel and perceive.
Sapient = ability to use logic and judgement.
Not quite the same, see? Of course, movies and TV shows fuck it up all the time. The corollary to that: the next generation grows up absorbing that misinformation. Artificial intelligences always 'become sentient' when sapience is what they really obtain.
Most animals are sentient.
I believe this is the basis for discouraging/banning animal cruelty. People who tortures animals may smile when they greet you on the sidewalk but imagine how they would treat a human who ended up at their mercy.
A dog can feel pain, happiness, affection, etc. They can perceive their environment and act upon it, chasing squirrels and such.
Most animals aren't sapient.
A dog can't recognize that the postman isn't a dangerous trespasser. You can train one not to attack, but s/he won't know the Why's.
There are a lot of interesting studies that are going into determining which animals are sapient. Some of them exhibit signs of sapience, did you know?
As far as we know, civilization requires sapience. Sentient animals can thrive and exhibit social behaviors, but they'll never have laws.
Imagine that we were space explorers. If we found a planet with nothing but single-celled organisms, we would say we found life, but neither sentient nor sapient.
If we went to another and found furry mammals that scurry into burrows when we approach, 'We have sentient life.' They might actually be sapient, too, but we have no evidence of that, yet.
Moving on, we land and a group of creatures wearing stitched clothes come out of wooden buildings to look at us. I think it's safe to say these may be sapient based on their capacity to make clothes, use tools, and design shelters.
As far as communication, there's a school of thought, which I subscribe to, that believes sapient creatures would need to have some method of communication in order to have any kind of cohesive bond between individuals. Right now we think it would have to be verbal and written like ours, but it really isn't beyond imagination that a race of sapient creatures could exist which use telepathy or elcor-like scent-based methods, is it?