The Farap crossing, about 30km north of Chardzhou along the Amu Darya, is the main overland border point between Turkmenistan and Uzbekistan. The crossing connects to Alat on the Uzbek side, and from there to Bukhara, which is the usual destination for travelers heading east. By Central Asian standards this crossing is reasonably functional: expect a thorough search of bags and vehicle on both sides, customs forms in Turkmen and Russian, and a total processing time of one to three hours depending on traffic and the mood of the officials.
The crossing is open to travelers on valid visas for both countries, but remember that Turkmenistan's visa requirements are among the most restrictive in the world — independent tourism is technically permitted only via organized tour, and most visitors arrive with a Letter of Invitation arranged through a registered agency. Transit visas, which allow a few days to cross the country, are somewhat easier to obtain but still require advance arrangement. If your documents are in order, the crossing itself is not problematic; if there are any questions about your letter or visa, resolve them in Ashgabat rather than at the border, where options are limited.