keysends are directly between lightning nodes that allow for accepting keysend payments. You're basically paying to a PUBKEY as desination

lnurl is for sending to an ACCOUNT on a lightning node. Here you've got a username@server identity and talking to a web server requesting it to provide you an invoice to pay based on parameters given.

Keysend: How to find a node pubkey to send invoiceless sats to

Lnurl: How to find an actual invoice to send sats to

A realworld analogy of paying a contractor for work: you can either request an invoice with all the details of the work and payment destination & pay that (lnurl), or you can skip the invoice, swing by their home (pubkey) and just drop off some cash (keysend)