LTE-A is Category 6 LTE. This means it is LTE that supports technologies such as carrier aggregation and multiple-input and multiple-output antenna designs, allowing greater data speeds.
Carrier aggregation refers to the ability of the network and a device to combine the capacity a network has that may be scattered over the radio frequency spectrum into a single channel.
For example, a mobile operator in South Africa may have spectrum in the 900MHz, 1,800MHz, and 2,100/1,900MHz frequency bands.
When unsupported phones connected to a network, they would connect in one of these frequency bands and only have access to its maximum bandwidth, which was always smaller than 20MHz.
With carrier aggregation, bandwidth across multiple frequencies can be combined to give devices access to chunks of bandwidth larger than 20MHz, translating into faster speeds.