Link Aggregation is bundling two or more links as one logical link for increasing bandwidth and availability. This can be done by one of the two protocols . One of them is the Cisco proprietary PAgP and the other is standard based LACP. PAgP is the Port Aggregation Protocol. And the LACP is the Link Aggregation Control Protocol. These two protocols does not interoperate each other. You can use only one of them to configure link aggragation. You can also check wiki definion, here.
Here, we will focus to the standard based, Link Aggregation Control Protocol. In another lesson, we will learn How to Configure LLDP on Cisco , Juniper LLDP Configuration and Nokia LLDP Configuration. The last Cisco devices are already use LACP instead of PagP.
There is also a limitation about ports in the bundle. In Cisco platforms the ports in a bundle is limited by 8ports . This can be change in some platforms. For example in Cisco 10000 series, only 4 port is supported in a bundle.
This bundle is called Link Aggregation(LAG) generally.
You can also view the below LACP Configuration Lessons for different vendors: