Carrier Aggregation is a technology to combine two or more carriers into one data channel to enhance the data capacity. It is possible to combine carriers in the same or different frequency bands. This is explained in the following figure.
The easiest way to aggregate carriers is to take two or more contiguous channels within the same frequency band (intra-band, contiguous). It is also possible to combine two or more non-contiguous channels within the same band (intra-band, non-contiguous). The most challenging form is to aggregate two or more carriers from different frequency bands (intra-band; non-contiguous).
Carrier aggregation technology is implemented in most modern mobile communication systems, like LTE and WiMax. In IEEE 802.11 (Wi-Fi) it is possible to combine two contiguous channels into one big data channels. In IEEE 802.11 terminology this is called channel bonding.