Official docs:
http://www.juniper.net/techpubs/software/junos-security/junos-security10.2/junos-security-swconfig-interfaces/aggregated-ethernet-overview-section.html#aggregated-ethernet-overview-section
http://www.juniper.net/techpubs/software/junos-security/junos-security10.2/junos-security-swconfig-interfaces/understand-cc-reth-interface-lags-section.html
Relation between the two:
In chassis cluster mode, aggregated Ethernet interfaces (ae) and redundant Ethernet interfaces (reth) coexist. Because the functionality of a redundant Ethernet interface relies on the JUNOS aggregated Ethernet framework, you can think of it as a special aggregated Ethernet interface.
src: http://www.juniper.net/techpubs/software/junos-security/junos-security10.2/junos-security-swconfig-interfaces-and-routing/understand-lacp-in-cc-mode-section.html
Simple Explanation:
Aggregated Ethernet Interface(ae)
http://jsrx.juniperwiki.com/index.php?title=Aggregate_Ethernet
Aggregate Ethernet (ae) is a way to aggregate ethernet interfaces (duh!). In laymans terms, you can bundle multiple links together to function as a single logical high bandwidth link.
Redundant Ethernet Interface(reth)
- An interface bundle made up of two or more Ethernet links, equally from each node in the cluster to form a single logical interface, acting as Active- Passive pair
- Used in a cluster where userspace daemon JSRPD determines the active member
- One IP Address, One vMAC . owned by node with active member
- RETH must be associated to an RG in the cluster