Yes, I have comprehensive experience of this much misunderstood topic.
Most tablets in the low price range have usb capability extending to use of flash memory stick, external keyboard and 3G dongle.
This comes via the mini usb socket which has OTG on-the-go capability.
The socket can be used in the normal way to a computer for download of files. In this case you use a cable which is a normal full-size male usb to the computer and mini usb to the tablet. The cable carries power FROM computer TO tablet. The mini plug has FOUR used pins for supply and data.
The otg socket can also be used via the CORRECT adaptor to convey power FROM tablet TO peripheral. In this case it is a FIVE pin mini usb plug with pins 4 and 5 (nomenclatures differ) linked together within the adapter. The link tells the tablet it has to supply power (not receive it).
The CORRECT otg adapter has the five-pin-with-link mini usb otg plug on one end and a full size female usb on the other end to accept the peripheral. The adapter may be solid or it may be a short cable.
BEWARE - There are a whole lot of dud otg adaptors on the market. They have the wrong mini usb plug and do not have pins 4 and 5 linked. I made a test rig to check continuity and isolation of otg adaptors, occasional items had line-to-line shorts, over half had the wrong mini usb plug, and relatively few actually worked. One pair of right and left hand connectors included one ok and one dud. Quite a lot of those shipped on the slow boat from the orient were dud. Most of those from the uk tributary of a South American river were ok.