You are mentioning two things. 1) yes, they send send signals, and their "inverse" over a pair of cables. That's guaranteed by the use of transformers on your network card/network switch. Current that the transformer sends in one line of a pair, has to be returned in the other pair.
On an old 10MBit or 100MBit/s card (10BaseT, 100BaseT) on each device there will be one pair to send data out, and one to receive data. This uses up 4 pins on your RJ45 connector, the other 2 are unused. The pair that is used on your network card to send out data are pins 1 and 2. The pair that is used on your network card to receive data is on pins 3 and 6.
The 2nd thing you mention are the two standards TIA/EIA-568 assigns colors to pins, and they have a version A and B where the colours of pair 1/2 and pair 3/6 are swapped.
Hence, when you have a cable where one side is wired according to "A" and the other is wired according to "B" you have something that, in the old days, was called a cross-cable, which you could use to directly connecto two computers with each other, without having a hub or switch in between. Which is very useful, especially if you'd connect e.g. two switches in two buildings with each other.
But because miswirings regarding the two color schemes were so frequent, and connecting routers/switches/computers directly using "straight" cables was so convenient, most devices manufactured after the 90s can swap these pairs internally automatically. Hence no matter if you have a strait or cross-cable, or a miswired 568A/B installation computers will just function fine.
With 1000BaseT (Gigabit-Ethernet) data is transmitted and received on all pairs simultaneously, and also this miswiring is automatically detected and taken into account.
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You are mentioning two things. 1) yes, they send send signals, and their "inverse" over a pair of cables. That's guaranteed by the use of transformers on your network card/network switch. Current that the transformer sends in one line of a pair, has to be returned in the other pair.
https://www.google.de/search?q=ethernet+transformer&tbm=isch
On an old 10MBit or 100MBit/s card (10BaseT, 100BaseT) on each device there will be one pair to send data out, and one to receive data. This uses up 4 pins on your RJ45 connector, the other 2 are unused. The pair that is used on your network card to send out data are pins 1 and 2. The pair that is used on your network card to receive data is on pins 3 and 6.
The 2nd thing you mention are the two standards TIA/EIA-568 assigns colors to pins, and they have a version A and B where the colours of pair 1/2 and pair 3/6 are swapped.
https://en.wikipedia.org/wiki/TIA/EIA-568
Hence, when you have a cable where one side is wired according to "A" and the other is wired according to "B" you have something that, in the old days, was called a cross-cable, which you could use to directly connecto two computers with each other, without having a hub or switch in between. Which is very useful, especially if you'd connect e.g. two switches in two buildings with each other.
But because miswirings regarding the two color schemes were so frequent, and connecting routers/switches/computers directly using "straight" cables was so convenient, most devices manufactured after the 90s can swap these pairs internally automatically. Hence no matter if you have a strait or cross-cable, or a miswired 568A/B installation computers will just function fine.With 1000BaseT (Gigabit-Ethernet) data is transmitted and received on all pairs simultaneously, and also this miswiring is automatically detected and taken into account.