efinitions
Triggered Send Definition - A triggered email is a message that Marketing Cloud sends to an individual subscriber in response to a subscriber action. For example, sending a confirmation message after a customer makes a purchase is a triggered email.
User Initiated Send - In Email Studio, create a user-initiated email message interaction to send to a user-initiated subscriber list or data extension in your program.
A user-defined email message interaction occurs when you manually start the send, while a triggered message interaction occurs in response to an action or API call. To put it in simpler words (TL/DR):
Triggered Send Definition - hit by API/Smart Capture to instantly send a 1:1 message. Usually used for Transactional messages (receipt, shipping, etc) and inside of Journies.
User Initiated Send - a 1:Many job that mimics a manual Guided Send, but is preset to allow a single click execution or to allow for repeatable scheduling inside of an Automation. (e.g. Monthly newsletter, daily update email, etc.)