These are devices that change the signal from optical to electrical (and vice versa).
Media converters are simple devices that convert an Ethernet signal from electrical to optical signal (1 or 2 fiber). The industrial media converters have an extended temperature range. Many of the media converters have fixed optical interfaces, but some can also be obtained with plug-in electronics (SFP) to accommodate different types of optical interfaces.
Switches allow you to connect multiple Ethernet lines before converting them to an optical signal. The switches exist with and without management. All uses plug-in electronics (SFP) that allows connection to different types of optical interfaces. Here too, the industrial versions have an extended temperature range.
Plugable electronics (SFP, XFP, etc.) are interchangeable devices that make the conversion itself from electrical to optical signal. They can be encoded to fit switches / routers from most vendors and can be delivered with most optical interfaces: standard (850, 1310, 1550 nm), CWDM and DWDM wavelengths, and data rates up to 40G.