OS X offers different folder view options that can be set by clicking the corresponding view button in the Finder toolbar: you can choose from list view, column view, Cover Flow, and icon view. Column view, one of the most popular, makes possible easy navigation of the filesystem using arrow keys.
While this view is convenient, in some cases its default column width may be too small for the file and folder names that you use, so when you navigate through your files a significant number of names will be truncated and condensed.
To manage these situations, you can drag separators to resize individual columns, double-click the separator, or right-click it to get a set of options for resizing columns; however, you'll need to do these things again each time you create a new Finder window.
If you want to change the default width of the Finder columns, then you can do so by holding the Option key when dragging one column. This will change the size of all columns in all Finder windows, and when you close and reopen the window, the column sizes should remain.
While this works for the Finder's column view, it will only work with this view, and not with the others such as Cover Flow.