But balance can also be less explicit, and require a little more work from the ol' visual cortex, such as the example of asymmetrical balance below:

Similar to unity, balance can convey order and unity of forms, but it can also help guide the eye as it flows through the content. Balance creates a feeling of order, whereas imbalance can create a feeling of unease or movement in an otherwise still composition.

The below examples exhibit some kind of balance or imbalance. 

Achieving balance or imbalance

Scale: Perhaps the most obvious way of working with and achieving balance is through the principles of scale and proportion. Due to our real-world experience with such things, forms that are larger in size feel heavier. A large form can thus be balanced with a larger number of smaller forms.

Colors and values: colors carry with them their own weight. Reds and purples may feel heavier than light blue or yellow. Value is also closely related to weight, as black often feels much heavier than, say, white or light grey. One can use colors to achieve balance, but it is often more interesting to create imbalance through contrast color and value weights.

Thickness: similar to scale, lines can be thick, and thus feel heavy. Likewise, other lines can be thinner and feel much lighter. This contrast in line weight is particularly important in the world of typography, where even a single letterform will require within it a balance of thick versus thin lines:

You'll likely find that many typefaces have bottom-heavy letterforms.