A thin value bet is a bet that is designed to get your opponent to call too frequently (as opposed to fold too frequently) but your hand is not very strong and is in fact worse than many of your opponents likely holdings.

When is it best to make thin value bets?

While there are exceptions, the river tends to be the best place to make a thin value bet. Marginally good hands are very vulnerable to being drawn out on, so they are better candidates for pot control on earlier streets.

How big should thin value bets be?

The expected value of a given betsize is determined by a simple math formula (although some of the variables you input will always be educated guesses).  There is legitimate concern that you could give away the strength of your hand if you are making different betsizes with slightly good hands than you would with a very strong hand. The fear is less that your opponent will stop calling with worse hands, and more that your opponent will stop calling with worse hands and start bluff raising you.

Are blocker bets and thin value bets the same thing?

Nope, in both we have a hand that is of marginal value, but among other things, blocker bets are small relative to pot size and are made out of position.

What happens if someone raises my thin value bets?

If a clever opponent starts raising your thin value bets, he may have legitimately trapped you with a good hand or he may be bluffing at your perceived weakness. I would default to folding and maybe not betting so thin next time. Calling or reraising (if there is enough money behind) can be a reasonable response, but at lower stakes this is not necessary to play winning poker and will likely result in fancy play syndrome instead of controlled aggression.