Downtown Disney is divided into three sections with West Side at one extreme, Marketplace at the other and Pleasure Island bridging the two.
West Side is home to Cirque du Soleil, DisneyQuest, Virgin Megastore and an AMC 24 screen cinema as well as four restaurants and several unique shops (Starabilias being my favourite). The restaurants are House of Blues, Wolfgang Puck Cafe, Bongo's Cuban Cafe and Planet Hollywood. I'd say Wolfgang Puck's is the best of the bunch, although it does get crazily busy. The House of Blues hosts a Gospel Brunch each Sunday and it's also a concert venue showcasing big names alongside less well known artists.
Pleasure Island is primarily a night-time entertainment district, although Disney recently announced plans to close the nightclubs to make way for more shops and dining

. Whilst the area is being redeveloped, the existing shops and restaurants at PI will remain open. Raglan Road is probably my favourite of the restaurants at DTD.
Its website does a far better job of describing it than I could.
The focus at Marketplace is squarely on shopping and dining (i.e. it doesn't have the dedicated entertainment options of the other two areas). It has more of the typical Disney feel, possibly due to the fact that Disney shops far outnumber the other stores there. Earl of Sandwich is great for a quick bite and Rainforest Cafe is a perennial favourite, although the Downtown Disney location gets a bit too busy for me - I prefer the one at Animal Kingdom.