Numbered means you give IP addresses to the connection interfaces.
When you use IP numbered you actually give the connection (a wire in PTP, or a frame relay network) its own IP network or subnet. So often this is look on as undesireable since it requires a perfectly good IP subnet to be wasted.
What many people do is use a subnet out of 192.168.0.0 for their WAN connections since it does not affect packets 'passing thru' the router.
exmaple:
====ethernet=====
|/192.168.10.1
+---------+
| e0 |
| Router A|
| w1 |
+---------+
|\192.168.230.1
|
| WAN connecton
|
|/192.168.230.2
+---------+
| w1 |
| Router B|
| e0 |
+---------+
|\10.145.23.1
====ethernet=====
Unnumbered means that the connection interfaces do not have IP addresses.
example:
====ethernet=====
|/192.168.10.1
+---------+
| e0 |
| Router A|
| w1 |
+---------+
|
|
| WAN connecton
|
|
+---------+
| w1 |
| Router B|
| e0 |
+---------+
|\10.145.23.1
====ethernet=====