Addresses in Bangkok use the Thai addressing system, which may be a little confusing to the uninitiated. Large roads such as Silom or Sukhumvit are thanon (ถนน), often abbreviated Th or glossed "Road/Avenue", while the side streets branching off from them are called soi (ซอย). Sois are numbered, with even numbers on one side and odd ones on the other. Thus, an address like "25 Soi Sukhumvit 3" means the 25th building on the 3rd soi of Sukhumvit Road. While the soi numbers on each side will always advance upward, the numbers often do not advance evenly between sides - for example, Soi 55 could be across from soi 36. Many well-known sois have an additional name, which can be used instead of the number. Soi 3 is also known as "Soi Nana", so the address above might thus also be expressed as "25 Soi Nana". The extension /x is used for new streets created between existing streets, as seen in Sukhumvit's soi pattern 7, 7/1, 7/2, 9, 11. Note that some short alleys are called trok (ตรอก) instead of soi.
To make things a little more complex, some large sois like Soi Ekamai (Sukhumvit Soi 63) and Soi Ari (Phahonyothin Soi 7) have their own sois. In these cases an address like "Soi Ari 3" means "the 3rd soi off Soi Ari", and you may even spot addresses like "68/2 Soi Ekamai 4, Sukhumvit 63 Road", meaning "2nd house beside house 68, 4th soi off Ekamai, the 63rd soi of Sukhumvit". In many sois the house numbers are not simply increasing, but may spread around.
To further bewilder the tourist who doesn't read Thai, the renderings of Thai street names in the Latin alphabet are not consistent. The road running towards the (former) airport from the Victory Monument may be spelled Phahon Yothin or Pahon Yothin or Phahonyothin or Phaholyothin depending on which street sign or map you consult. It's all the same in Thai, of course -- only the romanisation varies.
And if that's not confusing enough, most of the larger streets tend to change names altogether every few kilometers. Sukhumvit is called Sukhumvit on one side of the tollway (roughly east), but it becomes Ploenchit just before you cross Thanon Witthayu (aka Wireless) going towards the river. Keep going just a few more streets and it becomes Thanon PraRam Neung (usually said as just Rama I) after you pass Thanon Ratchadamri. But if you were to turn right onto Ratchadamri, in just a few blocks you'll find yourself on Thanon Ratchaprarop (past Petchaburi, aka New Phetburi, which is called Phitsanulok closer to the river). Got it?
But wait, there's logic to these name changes: most of them are neighborhoods. It wouldn't make sense to call the road Sukhumvit if it's no longer running through the Sukhumvit area, would it? Thus, Sukhumvit becomes Ploenchit where it runs though the Ploenchit area. It's when you're able to grasp the city in terms of its neighborhoods that it both becomes more navigable and more charming. Likewise, Pratunam and Chatuchak are much more than just markets; they're boroughs, each with its own distinct character.
Related to this last point, compass directions are not widely used by Thais to navigate in Bangkok. That's probably because they aren't very useful: the city's darwinistic layout, the changing street names, the winding river, and the lack of obvious landmarks all conspire to confuse your internal compass. Thus, asking for directions in terms of "is that west from here?" will probably earn you little more than a confused look from a local. You're better off to familiarize yourself with the neighborhoods and navigate to and from them. "How do I get to Thonglor?" will get you there faster than asking for directions to Sukhumvit Soi 55.