Silom is south of Sukhumvit and more upmarket, having some of the city's best hotels and an altogether nicer atmosphere. Silom and Sathorn roads are interesting and non-sleazy by day and come alive at night with an array of excellent restaurants on the main street and the infamous Patpong market and sex area.
Rattanakosin is actually an island in western Bangkok, hemmed in by a bend in the Chao Phraya River to the west and canals to the east. This is old Bangkok and consists of touristy areas around the Grand Palace and Wat Pho and the nearby backpacker haven of Khao San Road.
Yaowarat and Pahurat
These cultured areas of Bangkok are home to the city's Chinatown (Yaowarat Road) and the nearby Indian community of Pahurat Road. It is a fun area to be by day, with some great eating and market shopping. Night-time in Chinatown is fairly slow.
This watery part of Bangkok lies on the west bank of the Chao Phraya River, and although having many intriguing attractions, it's not all that touristy and is a real boon for those who don't like crowds. It is a lot less developed than central Bangkok but is pleasant by day and you can get on a tour of the canals and visit floating markets.
This quieter area in eastern Bangkok, near Sukhumvit and Silom roads, features Wireless Road, where many foreign embassies are based, and Sarasin Road. It is home to the tranquil Lumpini Park and many high profile shrines, including that of the famous Erawan Shrine. Chidlom is also loaded with classy shopping malls and department stores.