Roads often vary in appearance but they all exist as thoroughfares to connect two points, whether close or distant. Sometimes roads run between two separate towns, but they can also exist within a city or town. They tend to be wider than streets, with multiple lanes in each direction, allowing more cars to travel on them at quicker rates.
Avenues are not dissimilar to roads, but are usually smaller versions of roads. Most avenues have trees that run alongside them, which can run the entire route of the avenue or be interspersed throughout. In North America, avenues run through cities laid out as right angles to a grid. Avenues typically have businesses lining them, but not houses.
Streets are small and are in cities and towns. While it is not a requirement for streets to be entirely residential, it is much more likely to see houses on streets than commercial businesses. Unlike a road, a street doesn't necessarily connect two places and can sometimes lead to dead ends. Streets also typically have sidewalks on them.
Boulevards are a lot like avenues in that trees generally line them. However, boulevards are more like promenades -- designed for walking. Usually a grassy or landscaped strip exists between the two opposite directions of traffic, in the center of a boulevard, which serves to separate the traffic.
