Difference between wifi router and access point?

Access Point is just a bridge between wired and wireless networks. It only operates in 1st and 2nd layer. It doesn't perform any 3rd-layer operations (routing, NAT-ing, IP filtering...)

For the most part the two are used interchangeably, however, technically there is a difference and for the most part consumers only use WiFi Routers.

A WiFi router is a WiFi access point with a built in router. The router allows you to connect multiple computers to the network using a single IP address, typically provided by your ISP.

A WiFi access point is typically used only in enterprise networks where they have a larger router which routes their entire network and the access point only acts as a gateway between the wired and wireless networks.

The access point acts as an ethernet switch, has one Ethernet port and allows WiFi stations to access the same LAN the access point is in.

The router will typically have 4 ethernet ports, 1 special ethernet port called "internet" and will allow creating a new subnet for the Wifi, with DHCP, acting as the default gateway for this subnet