Traditionally, because strict adherence to the ACID semantics can be expensive, database systems provide a range of isolation models, trading off weaker consistency/isolation guarantees for performance.

Three isolation models are supported in WiredTiger, from weaker to stronger:

read-uncommitted: Transactions can see changes made by other transactions before those transactions are committed. Dirty reads, non-repeatable reads and phantoms are possible.
read-committed: Transactions cannot see changes made by other transactions before those transactions are committed. Dirty reads are not possible; non-repeatable reads and phantoms are possible. Committed changes from concurrent transactions become visible when no cursor is positioned in the read-committed transaction.
snapshot: Transactions read the versions of records committed before the transaction started. Dirty reads and non-repeatable reads are not possible; phantoms are possible. Snapshot isolation is the default isolation level, and all updates must be done using snapshot isolation.

Snapshot isolation is a strong guarantee, but does not always guarantee behavior equivalent to a single-threaded execution of the transactions. (The slightly stronger model that does is known as serializable isolation.) Given two concurrent transactions T1 and T2 running under snapshot isolation, if T1 reads data items updated by T2 and T2 reads data items updated by T1, but the data they update does not overlap, both may commit. But because each read the data from before they both started, not the other's output, the execution is not equivalent to either running strictly before the other and the resulting state may be one that no serial execution could produce. This behavior is called write skew.