The differences between an H-beam and an I-beam are very slight. The two beams look very similar in construction and are often called the same thing -- a W-beam or wide-flange beam. The beams are often used for different types of construction or different parts of the structure.
Both the H-beam and I-beam have top and bottom flanges. The flanges on an H-beam are longer and stick out farther from the center web. The flanges on an I-beam are shorter and not as wide. The distance from the end of the flange to the center web is shorter on an I-beam than the same measurement on an H-beam flange.
Another difference between an H-beam and I-beam is the fabrication method used to make the beams. The I-beam is fabricated by milling or rolling the steel. The size of the I-beam is limited by the capacity of the milling equipment, which this is why I-beams have smaller flanges. H-beams are built up rather than milled, so they can be made any height and width.
Since I-beams are milled or rolled, the web and flanges have a bevel where the three pieces come together and look like one piece. H-beams have the top and bottom flanges attached to the center web by welding or riveting them together. You can actually see that the H-beam is made of three different metal plates.
H-beams are useful for longer spans than I-beams because the H-beam can be fabricated to any size. I-beams are good for spans of 33 feet to 100 feet because of the size limitation. H-beams can be used for spans up to 330 feet.