Commend mostly means the same thing as praise, but is more formal - both the word and the meaning. If you commend someone, you make a formal declaration praising their good qualities. Recommend means what alina said, but is not the same thing as commend.

The dictionary definition of commend is "to praise". They mean exactly the same thing and probably just have different roots. Like Mike said, a commendation is usually a more formal endorsement and is used less commonly than praise. But, they could also be used interchangeably.

Praise= saying, that sth/sb is good; Commend= telling somebody that sth/sth is good and she/he should try it. Example: I am praising this book = I like this book, it's good. I (re)commend this book = This book is better than all the others, YOU should buy it and read it. You usually recommend sth to somebody and just praising sth. Understood?