Deciding how to structure a client’s account into campaigns means that you need to understand what settings can be changed at each level.
Splitting accounts into more than one campaign helps offer greater flexibility in terms of allocating the budget…. BUT if there isn’t much budget to allocate look out! You can’t spread it too thin and might have to begin with just one campaign. For example if clicks cost $5 and your daily budget for the campaign is $8 (because it has to share money with other campaigns) then the campaigns just won’t run properly.
The AdWords Blog has some an examples on the basics of structuring campaigns and it looks pretty good, take a look here