Each object within X - a Tweet, Direct Message, User, List, and so on - has a unique ID. At the very beginning of the platform, these IDs were small enough numbers that they could be generated sequentially. Over time, to accommodate growth, the IDs moved from being 32-bit, to 64-bit. Today, X IDs are unique 64-bit unsigned integers, which are based on time, instead of being sequential. The full ID is composed of a timestamp, a worker number, and a sequence number. X developed an internal service known as “Snowflake” in order to consistently generate these IDs (read more about this on the X blog). Numbers as large as 64-bits can cause issues with programming languages that represent integers with fewer than 64-bits. An example of this is JavaScript, where integers are limited to 53-bits in size. In order to provide a workaround for this, in the original designs of the X API (v1/1.1), ID values were returned in two formats: both as integers, and as strings. 
{"id": 10765432100123456789, "id_str": "10765432100123456789"}
If you run the command (10765432100123456789).toString() in a browser JavaScript console, the result will be "10765432100123458000" - the 64-bit integer loses accuracy as a result of the translation (this is sometimes called “munging” - a destructive change to a piece of data). In X APIs up to version 1.1, you should always use the string representation of the number to avoid losing accuracy. In newer versions of the API, all large integer values are represented as strings by default.