A cryptocurrency account is a collection of transactions sent to addresses that belong to that account. An account also provides an extra layer of privacy because transactions sent from different accounts in the same wallet never mix, unlike transactions sent from the same account.
Like other hierarchical deterministic wallets, Trezor uses account structure to organize transactions. The number of accounts supported by Trezor is not limited, but in Trezor Wallet, the number of accounts per cryptocurrency is limited to 10.
See also: User manual:Labeling
Standard vs. legacy accounts 
Standard accounts are the default acccounts available in Trezor Wallet for all the supported cryptocurrencies. Bitcoin, Litecoin, Vertcoin, DigiByte and Bitcoin Gold standard accounts are accounts with SegWit addresses enabled, meaning they use SegWit by default. A transaction using SegWit is faster and cheaper.
Legacy accounts are only available for cryptocurrencies with SegWit enabled, and for hold transactions of the previously standard type that have been made obsolete (without SegWit). Although it is recommended for various reasons to use only standard accounts, legacy accounts are a valid means of holding funds and making transactions.