According to M-theory, which unites the five superstring theories, there are exactly 11 alternate spatial dimensions that exist in parallel with our own physical reality. By shifting in and out of phase, it is possible to experience these alternate dimensions in varying degrees by seeing, communicating, touching, or fully interacting with individuals or objects in another dimension. This level of interaction is not always reciprocal, however. An individual may see or hear those in another phase while himself appearing to be invisible to those in the alternate dimension. SG-1 has frequently encountered alien races and technologies that have phase-shifting capabilities, including the Reetou, the Tollan, and several System Lords among the Goa'uld. The crystal skull allowed transport to an alternate phase, and Ancient technologies such as the inter-dimensional device from P9X-391, the Sodan cloak, and Merlin's phase-shift device have also permitted phase shifting between alternate dimensions.
M-theory is not to be confused with the multiverse theory which posits the existence of an infinite number of parallel universes, each existing and evolving concurrently with our own.
Cross Reference: Alternate Dimension, Multiverse Theory
Episode Reference: Arthur's Mantle, The Road Not Taken