PostgreSQL and ACID properties support

Hello,

I would like to know to what extent PostgreSQL includes support for the ACID
properties of transactions (Atomicity, Consistency, Isolation, and Durability).
In particular, does PostgreSQL include constructs to guarantee that a sequence
of SQL statements is executed atomically (i.e., either all execute to completion
or none of them do). Also, with regards to isolation, what is the status of
PostgreSQL's implementation with respect to providing the option of concurrency
of transactions as opposed to the operations taking place serially? Also, does
PostgreSQL fully support SQL99 CHECK constraints? And if the system crashes...
is the database guaranteed to remain in a consistent state?

Thanks,

Neil