Walls are part of buildings, and can be added or removed in Build Mode. Walls define rooms, and keep the inside in and the outside out - which is particularly important with The Sims 2: Seasons and The Sims 3: Seasons. Some items, such as paintings, toilets, some sinks, fire and burglar alarms need to be placed against walls. Walls can also be built diagonally. Diagonal walls can give a house more interesting shapes, but it may not be possible to place some objects on or against them. In The Sims 2, The Sims 3 and The Sims 4, windows and doors can be placed on diagonal walls, which was not possible in The Sims.
The Sims 2 allows either 2000 or 2100 wall segments per lot  before the game will not allow a player to add any more - half-walls and fences are not included in this limit, however the game will not allow a player to add half-walls once the limit has been reached (though it is possible to add additional half-walls by erasing a single wall segment, placing half-walls, then replacing the wall). This limit has either been substantially increased or eliminated altogether in The Sims 3, though the upper limit (if any) is unknown.
Walls can be covered by paint, wallpaper, brick, stone, wood panels and siding, etc. to increase the mood of Sims. In The Sims 3, walls that are not covered will give Sims negative moodlets; in earlier games, uncovered walls will reduce the Room/Environment score.
Half Walls are a special kind of wall, introduced in The Sims 2: Nightlife, Patch 17 of The Sims 3 and Patch 21 of The Sims 4. Half Walls are comparable in height to fences or come in five different sizes[TS4], ranging from short to very tall. They are completely solid, and can be painted just like a full wall, but only use the bottom part of the wall paint. Its covered top also have different colors in The Sims 2 and can be recolored using Create a Style in The Sims 3. Fireplaces, doors and windows, and other wall objects cannot be placed on half walls, but fence post lights can be put on the posts of the half-walls. There are different kinds of half-walls that have different tops, designs on the tops[TS2:NL] and colors[TS2:NL].