This may seem to be a basic topic, but it looks like many people are still confused by the difference between those two concepts. Let us clear this confusion at once!
To begin with, why whould anyone need to run Multilink PPP (MLPPP or MLP) with Interleaving over Frame-Relay? Well, back in days, when Frame-Relay and ATM were really popular, there was a need to interwork the two technologies: that is, transparently pass encapsulated packets between FR and ATM PVCs. (This is similar in concept with modern L2 VPN interworking, however it was specific to ATM and Frame-Relay). Let's imagine a situation where we have slow ATM and Frame-Relay links, used to transport a mix of VoIP and data traffic. As we know, some sort of fragmentation and interleaving scheme should be implemented, in order to keep voice quality under control. Since there was no fragmentation scheme common to both ATM and Frame-Relay, people came with idea to run PPP (yet another L2 tech) over Frame-Relay and ATM PVCs and use PPP multilink and interleave feature to implement fragmentation. (Actually there was no good scheme for native fragmentation and interleaving with VoIP over ATM - the cell mode technology - how ironic!)