By plane
Kabul International Airport IATA: KBL in Kabul is the main entry point to the country. In late 2008, the barely functioning old terminal was refurbished and is now being used for domestic flights, while the brand new Japanese-constructed terminal is up and running and fielding international flights.
The national carrier, Ariana Afghan Airlines (http://www.flyariana.com/), is flying with a small fleet of about 14 Airbuses and Boeings plus Antonovs. They have daily flights from Dubai, and periodic flights from Frankfurt, Islamabad, Delhi, Istanbul, Baku and Tehran. Ariana is particularly bad at keeping to schedules, flights can be cancelled or delayed without notice.
A better option is the independent operator Kam Air (http://flykamair.com/), which has twice daily flights from Dubai, twice weekly flights from Delhi and weekly flight from Almaty, Istanbul and Mashad. Some of the flights on the Dubai to Kabul route stop in Herat if you'd prefer to enter the country there. Pamir Airways (http://www.pamirairways.af/) is a new private airline that offers daily flights between Kabul and Dubai $330 inbound, $210 outbound, some stopping in Herat. Safi Air also provides flights between Dubai and Kabul. They are the only safety accredited airline in Afghanistan. Safi is the only Afghan airline allowed to fly into Europe and has direct flights to Frankfurt, Germany. The service is good and planes are sound. Staff are professional.
Air Arabia (http://www.airarabia.com/) flies 4 times per week from Sharjah - however they have currently suspended operations. Pakistan International Airlines PIA (http://www.piac.com.pk/) flies 4 times per week from Islamabad and 1 time per week from Peshawar to Kabul. Another route in may be via through Tehran or Mashad in Iran. Iran Air (http://www.iranair.nl/) has periodic flights from Tehran to Kabul. Air India (http://www.airindia.com/) operates six flights a week from Delhi to Kabul. Spicejet, an Indian low-cost carrier flies three times to Kabul directly from Delhi Tue, Thu, Sat. Turkish Airlines also began flights between Kabul and Istanbul in 2011.
Flights to other cities such as Mazar-e Sharif may be available if you can hook up with the charter company PACTEC (http://www.pactec.org/avi...) however seating is very limited.
By bus
Buses run regularly between Jalalabad and Peshawar, Pakistan. Also, between Herat and Mashad, Iran. Afghani buses are thoroughly checked by Iranian border police for possible drugs, so expect delays.
By car
WARNING: The famous Khyber Pass is currently closed to anyone except Afghans or Pakistanis. Some travel blogs/forums claim that hiding in a vehicle and bribing the border guards works, but doing so is very risky and could lead to imprisonment. Even more risky, however, is the threat from Taliban near the pass, who have been known to kill/kidnap Westerners. You are strongly discouraged from passing through the Khyber Pass. May 2009
There are a number of roads into Afghanistan:
From Peshawar, Pakistan via the Khyber Pass to Jalalabad, in the East.
From Quetta, Pakistan to Kandahar, in the South.
From Mashad, Iran to Herat, in the West.
From Uzbekistan to Mazar-e Sharif, in the North.
From Tajikistan to Kunduz, in the North-West.
As of mid-2009, none of these routes can be considered safe. The Khyber and the Quetta to Kandahar route are particularly dangerous.