The difference is in how you specify the font and text size. You can enter layout mode, select the field layout object and specify a font and font size. In browse mode, you can select the text in the field and use the formatting tool bar to specify a font and font size. Also, if you paste text into a field, the font and fontsize is also pasted with the text. This second method specifies the font and font size for just the data of the record that where you did this. If you specify the font and font size for the field object, it applies to all records in your layout's table.
But: The text styles that you specify in browse mode or paste with the text from the clipboard supercedes the styles specified for the layout's field object. If your remove the text styles applied to the text in the field, the layout based style will then control the appearance of the text in the field.
Often, it's useful to set up text fields with this auto-enter calculation: TextFormatRemove ( self ) and then clear the "do not replace existing value..." check box. That causes the field to automatically remove any pasted text styles and browse mode applied text styles so that the layout mode specified text style will uniformly control the appearance of the text in the field.