The newest standard transaction types are P2WPKH (pay-to-witness-public-key-hash) and P2WSH (pay-to-witness-script-hash). Each of these can be expressed in two ways, native SegWit or embedded P2SH. While the former is smaller, it requires that the sending client supports bech32 addresses (see https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki). AFAIK, only Electrum does at this point, so for backwards compatibility, SegWit addresses can be wrapped in a P2SH address--thus making them understandable by essentially all clients. For a more in-depth explanation, see BIP 141 (https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki).