Did you know that there are different HDB flat types? And within each flat type, there are varying models:

2 bedroom flat
Consists of 1 bedroom, kitchen, living room:

2 ‘I’ (Improved) 45 sqm/484sqft
2 ‘S’ (Standard) 41 sqm/441sqft (floor plans: 1)

3 bedroom flat
Consists of 2 bedrooms, kitchen, living room:

3 ‘A’ (Modified) 90sqm/969sqft
3 ‘NG’ (Modified) 83sqm/896sqft
3 ‘A’ 75sqm/807sqft
3 ‘NG’ (New Generation) 69sqm/743sqft (floor plans: 1)
(2 toilets, master bedroom with attached bathroom)
3 ‘I’ (Modified) 70sqm/750sqft
3 ‘S’ (Simplified) 65sqm/700sqft
3 ‘I’ (Improved) 60sqm/646sqft
(No attached bath, toilet and bath separated, no storeroom)
3 ‘STD’ (Standard) 54sqm/581sqft 
(No attached bathroom/storeroom. Upgraded units have extra utility room or toilet)

4 bedroom flat
Consists of 3 bedrooms, kitchen, living room.

4 ‘A’ 105sqm/1130sqft
(2 bathrooms, master bedroom with attached toilet, storeroom) (floor plans: 1)
4 ‘NG’ (New Generation) 92sqm/990sqft (floor plans: 1)

4 ‘S’ (Simplified) 85sqm/914sqft (floor plans: 1)
(2 bathrooms, master bedroom with attached toilet, storeroom)
4 ‘I’ (Improved) 83sqm/893sqft 
(toilet and bath separated, no storeroom)
4 ‘STD’ (Standard) 73sqm/786sqft

5 bedroom flat
Consists of 3 bedrooms, kitchen, dining room, living room, storeroom.
Master bedroom comes with attached bathroom.

5 ‘A’ 135sqm/1453sqft
5 ‘I’ 123sqm/1313 sqft (floor plans: 1, 2, 3, 4, 5)
5 ‘S’ 121sqm/1300sqft (floor plans: 1)

Executive flat

EA (Exec Apartment) 141sqm/1518sqft (single storey) (floor plans: 1)
EM (Exec Maisonette) 145sqm/1560sqft (double storey) (floor plans: 1)
(double story, 3 bedroom/bath upstairs, balcony+additional room)
MG (Multi-Generation) 165sqm/1776sqft

Miscellaneous

Miscell Flats (varies) – These are flats created by purchasing and joining 2 adjacent units. Such units are considered as one and have one address, utility meter and so on.

eg. a 3+3 is created by combining (2) 3-rm units and building a passageway through.

Not all flat types exist in each estate, and a 4 ‘A’ layout in one estate might differ from another.

Also, floor area are estimates as newer flats are generally smaller, while upgrading of older flats would have added additional rooms/balcony/toilet.

source: http://singaporewatch.org/?p=582