
The best learning resource for this would be a good introduction on typography – probably the seminal classic by Bringhurst (see http://webtypography.net for a good roundup applied to the web), though e. g. Spiekermann's ‘Stop Stealing Sheep…’ is not bad for starters, either – and on design grids (see my answer here on UXexchange).
When designing grids you are mostly using a basic module (proportions ideally defined by working from the content outwards) that all content is fitted to (i.e. multiples of it).
Vertical and horizontal margins between blocks are in most cases different. Vertical whitespace is often oriented on the baseline grid (see e.g. Bringhurst, again). Using one is highly recommended to achieve at a unifying vertical rhythm.
The minimum amount of horizontal whitespace, i. e. primarily the separation between columns of body text (gutter), is governed by Gestalt psychology with font size, line spacing, and line width as main influencing factors. You should place text blocks far enough from another such that your recipients will be able to see them as distinct units of their own. A traditional rule of thumb would make the gutter at least 1.5 ems wide in order to appear significantly wider than any possible whitespace within a line of text. On the screen, good line spacing tends to be a little bit wider than in traditional print, though. Hence you will probably need a little bit more than that. Using the same value as your baseline grid is a good guess to start with in most cases.
BTW: design standards – unless significantly backed up by ergonomics or cognitive psychology – are never standards in the more rigid sense of the word. You may violate any ‘standard’ as long as you know why you are doing it. 


What are the rules of thumb for margins in web design?
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