A wild Pokemon is easier to catch after you give razz berry. The increase of catch rate is uncertain, but it is expected to be about 15 percent. However, the wild Pokemon's escape rate is not decreased.
2. Razz Berry can be used again after failing to capture
The effect of Razz Berry cannot be accumulated. However, if you fail to catch a wild Pokemon after giving Razz Berry to it, The effect of the first Razz Berry is invalid and you can use Razz Berry again. If your Pokeball do not hit a wild Pokemon, you cannot use the second Razz Berry but the effect of the first Razz Berry is valid.
3. How to get Razz Berry
You can get Razz Berry from at trainer level 8
Once your trainer level reaches 8, you can get Razz Berry from PokeStops. Razz Berry is not purchasable at the in-app shop. While the CP of wild Pokemon is not high, you do not need to use Razz Berry. However, when you trainer level is more than 15, the CP of wild Pokemon become high and Razz Berry is necessary to catch.
4. New Razz Berries
New Razz Berries are expected to appear in Pokemon Go due to future update. How to get these berries and the effect of those are not announced.