Wifi - How does channel difference between transmitter and receiver affect the signal/data?
Signals from other channels are picked up as noise, lowering your signal-to-noise ratio (SNR). In principle, it could be picked up as data, but will almost always fail some very basic checks (correct preamble, destination MAC address, ...).

The channel mapping is built so 802.11b channels have sufficient distance to not influence each other. With 802.11g, the transmission bandwidth is wider, but the channel mapping was retained for compatibility purposes.

Interfering transmissions cause information to be lost, which is generally corrected for by a retransmission, decreasing efficiency of the system.

If any access points using the g protocol are around, it is generally best if everyone uses channels 1, 6 and 11 exclusively, as this avoids interference between adjacent channels, leaving only conflicts on the same channel, which the protocol is built to avoid even between different networks.