With the photograph open in Photoshop, click on Image from the menu and a drop down list appears. Select image size from this list.
This brings up a dialogue box with various dimensions and check boxes.
It's here where you can either resize or resample a picture.
Resizing allows you to adjust the number of pixels per inch and keeps the same amount of data by adjusting the dimensions to suit the new pixel setting.
Re-sampling keeps the same dimensions and removes or adds pixels to reach the new pixel setting you key in. Increasing the number of pixels is usually referred to as interpolation.