Today (2019-02-15), you must acquire a one-time-use Lightning invoice from a user in order to initiate a Lightning payment to them. Key Send is a technology currently in development that will allow any node to send a payment to any other (online) node without requiring an invoice.
In lnd version 0.7.0, Sphinx Send was renamed to Key Send.
Today, a payer must receive a unique invoice from a payee for each payment:
Many tools have been built that LND users can host on a webserver in order to dispense invoices to users:
GitHub - brndnhrbrt/ln-donate-node: A set-and-forget Bitcoin Lightning Network invoice generator
GitHub - michael1011/lightningtip: Get tips via the Lightning Network
With Key Send, these dynamic invoice generators can be replaced with a static string or QR code that anyone can pay to.
LND has work-in-progress code that will enable Key Send:
[WIP] multi: add new draft key send mode for spontaneous payments by Roasbeef · Pull Request #2455 · lightningnetwork/lnd
This work is incomplete, but if you build two LND nodes from source using the Roasbeef:new-eob-sphinx-send feature branch, these nodes are able to exchange Sphinx Send payments.