Though not an Aspect itself, an explanation of a Stellium is appropriate in this section as Stelliums are made up of Aspects -- Conjunctions, to be exact. A Stellium refers to three or more Planets that form a series of Conjunctions in an astrological chart, regardless of whether the Planets span into two Houses or if they are in different Signs or the same Sign. Whatever the combination may be, their combined energies create a particular focus, making a Stellium deserving of attention.
Within a Stellium, the power of the Planets is focused according to the Signs and Houses involved. For example, if a birth chart were to contain a Stellium of Mercury, Venus and Mars in Taurus in the Fourth House, this would create a tremendous focus of the steady, possessive, possession-oriented, sensual energy of Taurus. That heightened energy would manifest itself as related to communication (Mercury), love (Venus) and drive (Mars), as further related to home and family (Fourth House). Sound complicated? It's really not -- just think of it as a focal point of energy.
One more complication: A Planetary member of a Stellium is subject to the influence of aspects to other Planets in the same Stellium, even if the Orb is too wide to normally include that Planet. This is called aspect by association.