The Basics
A cryptographically signed container that details the transfer of Monero to a recipient (or recipients).
The parameters of a transaction contain one or more recipient addresses with corresponding amounts of funds and a ring size parameter that specifies the number of outputs bound to the transaction. A higher degree of obfuscation is possible with more used outputs, but that comes with a cost. The transaction fee will increase since a transaction gets more prominent with more outputs.
It is possible to form a transaction offline, which offers additional privacy benefits.
A transaction can be uniquely identified using an optional Transaction ID, usually represented by a 32-byte string (64 hexadecimal characters).
In-depth Information
Every transaction involves two keys: a public spend key, and a public view key. The transaction’s output destination is a one-time public key computed from these two keys.
When a wallet scans for incoming transactions, every transaction is scanned to see if it is for “you.” This only requires your private view key and your public spend key, and this check is immutable and cannot be faked. You cannot receive transactions and identify them without a corresponding private view key.
To spend the funds, you have to compute a one-time private spend key for that output; this is almost always done automatically by the Monero Wallet software.