What's the difference between `z-addr` and `t-addr` type addresses? I was playing with the zcash testnet and figured there are two different types of addresses available: z and t. Is this a privacy feature?

What's the difference between z-addr and t-addr type addresses?
This is now explained in the current Beta Guide:


  Zcash has two kinds of address: a z-addr is a fully private address
  that uses the zero-knowledge proving system to shield a transaction
  and balance privacy. A t-addr (aka "transparent address") is similar
  to a Bitcoin address.


An address can be created using:

zcash-cli getnewaddress    # t-addr, or
zcash-cli z_getnewaddress  # z-addr


t-addr is a 36 characters long string beginning with t.

z-addr has 96 characters and starts with z.