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A connector is more-or-less permanently attached to the end of a cable or to a hardware unit. There are two basic types of connectors used in serial communications: 1. DB types with pins (DB 9 or DB25) and 2. modular telephone-style connectors.
An adapter looks about like a connector but it has two ends with pins. It is just like a cable that is so short that there is no cable part left at all --just different connectors on each end is all that remains. The adapter just plugs in to two other connectors on each side of it. It allows two incompatible connectors to mate with each other by going in between them. Except that even for two connectors that will mate with battery such as dell C9553 battery , Dell CC156 Battery , Dell C5446 Battery , Dell D5552 Battery , Dell F5125 Battery , Dell Precision M6300 Battery , Dell G5252 Battery , Dell XP115 Battery , Dell Y4500 Battery , Dell Y4501 Battery , Dell Y4504 Battery , Dell F5132 Battery each other, an adapter may be used to connect the cable conductors together in other than straight-thru. Obviously, one may use a special cable (perhaps homemade) as a substitute for an adapter.
RJ11 are all the same size but may have 2, 4, or 6 conductors. If it has two conductors, it should be called a RJ11. If it has 4 conductors, some call it a RJ14. If it has 6 conductors, many call it a RJ12 (but a RJ12 per the phone company has only 4 conductors). Seems confusing but they are all the same size and differ mainly by the number of conductor contacts present.
A look-alike (almost) is a MMJ connector (6-conductor) used on later model VT (and other) terminals. It's sometimes referred to as a DEC-423 or a DEC RJ11. MMJ has an offset tab and is not compatible with RJ ones (unless the tab is cut off). However, some connectors have been made that are compatible with both MMJ and the RJ ones. Since MMJ connectors are both hard to find and may be expensive some people have forced a RJ (6 conductor) to fit MMJ by filing off the offset tab with a file.
The MMJ (DEC) pinout is: 1-DTR, 2-TxD, 3-TxD_Gnd, 4-RxD_Gnd, 5-RxD, 6-DSR. Cyclades Cyclom-8Ys RJ12 has: 1-DTR, 2-TxD, 3-Gnd, 4-CTS, 5-RxD, 6-DCD. Specialix IO8+ has: 1-DCD, 2-RxD, 3-DTR/RTS, 4-Gnd, 5-TxD, 6-CTS. The pins of the RJ (and MMJ) are numbered similar to the RJ45.