That service does more NLP level stuff - remove stop-words (a, the, an, etc.), tokenize text (eating becomes eat), keep only words that represent the core message of the text, I think that's about it
Neat, that's actually something I'll find useful in one of my personal projects. Guess I'll have to check it out.
We used Lucene (open source) in our information retrieval course and tokenizing (w/ removing stop words etc.) is one of the things it does. If you just want to experiment, that's also another option to look at if you like!
That's stuff I can hack together in 20 minutes using JS or PHP... Why on earth would you need a remote API for that?