There diferent aproaches to answer your question.

##1) On a already generated image, like a photo from a camera##

The ppi info on a image file is just a tiny little number inside the file. It is only meaningfull when you are thinking on printing the file. It answers just a question. "*Of thoose pixels you already have in your photo, how many you need on each phisical inch?*"

This produces diferent size in the **printed result**.

"*I want you to use 300 ppi on each inch*" or "*No I want a bigger print, just use now 150 on each inch*" This will make twice bigger print.

In both cases your original photo is exaclty the same.

##2) Before making a file on a raster image program##

If you create a new file on a program like Photoshop and you define it terms of pixels, you just have the previus case.

But if you define a new file in terms of a phisical dimension, for example a 8x10" image. The number you use on ppi affects the real file size.

If you define this 8x10" with 300 ppi you have a file (8x300)x(10x300) = 2400x3000 px.

But if you use 150 ppi you just have a (8x150)x(10x150) = 1200x1500px.

##3) Wich aproach should you use?##

In general terms I recomend defining a file in px, that way you can work on a file that you can handle, for example a digital paint. Some people thinks that if they just use a higher ppi they will get a higher quality, but sometimes they sacrifice the performance making a exagerated file, for example on a billboard.

In this post I posted some graphs regarding this issue:

http://graphicdesign.stackexchange.com/questions/45833/how-to-create-a-very-big-photo-like-for-a-billboard/45844#45844