The most commonly used overland crossing into Cambodia from Thailand is at Poipet, which lies just across the border from the Thai town of Aranyaprathet, 210km due east of Bangkok. It’s best to arm yourself in advance with an e-visa for Cambodia and to make the journey by regular public transport, but it’s also possible to buy a package all the way through to Siem Reap and to get a thirty-day visa on arrival at the border, though both of the latter options are more likely to open you up to possible scams, including a fake “Cambodian Consulate” in Aranyaprathet and rip-off currency exchange (it’s not compulsory to buy riel before entering Cambodia, despite what some touts may say). For further details, see wthaivisa.com for its visa-run forum; and wtalesofasia.com/cambodia-overland.htm for a very detailed description of the crossing and for advice on onward transport into Cambodia. Once you’ve walked across the border and entered Cambodia, it’s about two hours in a taxi or bus to reach Siem Reap, 150km away. If you have the deep misfortune of getting stuck in dusty, dirty Aranyaprathet, where local transport comes in the form of tuk-tuks, try the comfortable fan and a/c rooms at Inter Hotel at 108/7 Thanon Chatasingh.
From Bangkok, you can travel to Aranyaprathet Station, 4km from the border post, by train; you’ll need to catch the 5.55am if you want to get across the border the same day. Return trains depart Aranyaprathet at 6.35am and 1.35pm. Alternatively, take a bus from Bangkok’s Northern (Mo Chit) Bus Terminal to Aranyaprathet, or a faster, more expensive a/c minibus from Victory Monument. To reach Aranyaprathet from east-coast towns, the easiest route is to take a bus from Chanthaburi to the town of Sa Kaew, 130km to the northeast, and then change to one of the frequent buses for the 55km ride east to Aranyaprathet.
It’s also possible to buy a through ticket to Siem Reap from Trat and Ko Chang, or from Thanon Khao San in Bangkok, but this option is dogged by scams (including a visa “service charge”), takes much longer than doing it independently, and nearly always uses clapped-out buses or even pick-ups on the Cambodian side, despite the promised “luxury bus”.