What's the main difference between inter vlan and intra vlan?

in InterVlan, A Layer3 Switch is used to communicate the two vlans between each other, whicha layer2 switch can't do.

While Intravlan is used to communicate the two VLANs by means of A router for example, Router-on-a-Stick methode. 

InterVlan is a mechanism in which different Vlans can communicate to each other.IntraVlan is a mechanism in which let2  hosts in same VLAN and in different physical L2 switches to communicate to each other.

VLAN is a is a group of devices connected to administratively defined ports on a switch. VLAN's break up broadcast domains in layer2 switch network, but intervlan use to creat VLAN or VLAN.

for details please read this article

http://www.firewall.cx/networking-topics/vlan-networks/222-intervlan-routing.html