In SLIP/PPP, SLIP stands for Serial Line Internet Protocol, and PPP stands for Point to Point Protocol.
You can say that SLIP is used for delivering IP packets over dial-up lines, and PPP is used for transmitting the IP packets over serial lines.
Serial Line Internet Protocol (SLIP) or Serial Line IP (SLIP) was the first protocol for relaying IP packets over dial-up lines. It defines an encapsulation mechanism, but little else. There is no support for dynamic address assignment, link testing, or multiplexing different protocols over a single link.
SLIP has been largely supplanted by PPP (Point to Point Protocol).
PPP is the Internet Standard for transmission of IP packets over serial lines.
The Point-to-Point Protocol (PPP) is currently the best solution for dial-up Internet connections including ISDN.
PPP is a layered protocol, starting with a Link Control Protocol (LCP) for link establishment, configuration, and testing. Once the LCP is initialized one or many of several NCPs (Network Control Protocols) can be used to transport traffic for a particular protocol suite.
The IPCP (IP Control Protocol) permits the transport of IP packets over a PPP link.
PPP supports both synchronised and unsynchronised lines.