LTE Advanced refers to a series of updates to the LTE standard.
These updates address many areas, most notably network capacity and data speed.
The first version of LTE to be branded "LTE Advanced" comes in the form of phones supporting LTE Category 4. The main new feature of LTE Cat. 4 for users is Carrier Aggregation, which network operators can use to increase the top theoretical data rate to 150 Mbps (up from 100 Mbps with original LTE.)
See: Carrier Aggregation
However, LTE Cat. 4 phones are using the 3GPP release 8 version of the LTE standard. LTE Advanced takes another notable technology step with 3GPP release 10, which translates to phones of LTE Category 6 or greater.
Another key feature of LTE Advanced is improved support for small cells, which increase network density and thus network capacity.
See: Small Cell