A Starter Pack is an account upgrade that gives all characters in the account a few benefits. This applies to all characters already created and ones created in the future. Note that these benefits are useful even for VIP accounts.
Most common way to acquire one is through a give away done by gaming sites. These give aways are done in part to promote the game to readers of that site and in part to promote that site to players of Lotro and often require that the person creates an account with that specific gaming site.
The Starter Pack itself is a code that can be redeemed in-game at the LOTRO Store, following the same procedure as for the weekly store promotion.
In the bags of each character, a Bundle of the Giver will appear. Opening it will give the character: