The GPS error budget lists the errors that you have to account for when calculating your receiver's position.  Some of these errors can be estimated, thereby reducing their affect (like coupons for your food budget), and others must just be tolerated.

For GPS, different error sources are usually binned into different categories, such as Signal-In-Space (SIS), Atmosphere, Receiver, etc.  As you might expect, there are some differences of opinion as to which error goes into which category.  That doesn't matter for what we're discussing here though.  So, here's a typical error budget for GPS:

Note that the Ionospheric error results from mis-modeling by the receiver.  Also, some receiver's model tropospheric errors, others do not.  Caveat emptor.

The SIS and atmosphere errors are along the line-of-sight vector from the receiver's antenna to the GPS satellite.  Multipath error results from a ranging signal from a single satellite being reflected off of multiple surfaces and then back to the receiver - making the apparent range to the satellite longer.  Receiver errors typically come from noise induced by the GPS signal tracking process.  Note that other sources of RF interference are not listed here and can substantially increase your receiver's noise contribution. Other error budgets exist as well.

All of these error sources are dynamic and only a few can be modeled.  The ionospheric error is modeled in your receiver by a half-cosine model developed by John A (Jack) Klobuchar and accounts for roughly 50% of the effect of the ionosphere on the navigation signal. This model is specified in the GPS Receiver Interface Standard (IS) IS-GPS-200D. The remaining unmodeled ionospheric error is the largest remaining source of error for single-frequency (civilian) users of GPS.  Tropospheric errors may be modeled in your receiver as well, but no standard algorithm is defined in the IS.