Taxiways have distinct markings that indicate important differences in each taxiway. Basic taxiway markings are the edge lines and centerline, but the other markings you might see are probably equally important, if not more so.
Taxiway Edge Lines
A taxiway with two solid yellow lines running along the edge of the taxiway has a solid taxiway edge. A solid taxiway edge indicates the edge of the usable taxiway pavement and that the pavement abutting the taxiway is not safe for aircraft usage.
If the taxiway edge line is dashed, instead of solid, it indicates the adjoining pavement is usable for aircraft movement. Usually a dashed edge line abuts an apron or runup area.
Not all taxiways will have a centerline, but if one is present it will be a singular solid yellow line.
Enhanced Taxiway Centerlines
At some larger airports, the taxiway centerline is enhanced within 150 feet of the runway holding position sign/markings to warn the pilot they are approaching a runway and the associated mandatory hold short area. An enhanced taxiway centerline has two sets of parallel dashed lines alongside the normal centerline, one on each side.
Runway Holding Position Markings
Runway Holding Position Markings (or the runway hold short lines) are placed in conjunction with the Runway Holding Position Sign. These markings consist of two parallel solid yellow lines and two parallel dashed yellow lines running perpendicular to the taxiway centerline across the length of the taxiway.
Do not cross the runway hold short lines until cleared to do so by ATC, or in the case of an uncontrolled airport, until you have carefully cleared the area and announced your intentions on the common traffic advisory frequency!
Surface Painted Taxiway Markings
Information such as taxiway direction signs, location signs, and holding position signs are often painted on the taxiway pavement to supplement the associated signs, or in lieu of the signs if spacing does not allow for installation of such. Refer to Airport Signage: Taxiways for more information regarding signage.
ILS Critical Area
The beginning of the ILS critical area will be marked with a red sign with white lettering "ILS" (refer to Airport Signage: Other) as well as holding position markings on the ground. The markings are two parallel yellow lines split by perpendicular lines that span across the width of the taxiway. When advised by ATC that the area is active, pilot must no cross these markings until cleared to do so. Crossing this area without clearance will result in a pilot deviation and could catastrophically disrupt the instrument landing system.
When a taxiway is temporarily closed, a NOTAM (Notice to Air Missions) will be issued giving the date(s) and time(s) the taxiway will be closed. A notice may also be broadcast on the airport ATIS, if applicable. Temporarily closed taxiways will typically be blocked with lighted barricades or yellow "X's" to ensure pilots who forgot to check the NOTAMs don't endanger themselves or others.
Permanently closed taxiways will have associated lighting disconnected, and will be marked with large yellow "X's" at entry points and at 1,000' intervals along the length of the taxiway. They may also be blockaded or marked with "no entry" signs.
It is important to note that depending on the duration or cause of the closure, transient markings may not be in place. Always check NOTAMs, at your local airport and at unfamiliar airports!