Today's word is RES, which is a fifth-declension noun (gen. rei).
Latin meaning and usage: The range of meaning for the Latin word res is simply astounding; sometimes you can make do with the word "thing," but there is simply no English equivalent for the wide-ranging meanings of this Latin word. Take a read through the Lewis & Short dictionary entry to get a sense of the many possible English translations.
Latin word formation: You can find the word res used in many different expressions and compounds. One of the most famous, of course, is res publica, sometimes written respublica. The phrase quam ob rem can also be written as one word: quamobrem. You can also see the word res in the expression quare = quā rē? Be careful also with the verb refero; there is a typical compound re+fero, meaning "carry back, bring back, report," etc., but there is a different compound, rē+fert, meaning "it is somebody's business" - meā rēfert, "it is my business," tuā rēfert, "it is your business," etc.
English cognates and derivatives: You can see the Latin rē (= in rē) in the "re:" line used in the subject line of memos and letters. We also get the word "republic" (and its derivatives) from this Latin word.
Here are some examples of today's word in Latin sayings and proverbs; for more examples, see the page at the Scala Sapientiae, which also contains notes on some of the proverbs cited below: