You might guess from the name that coronal loops are structures in the corona that look like loops, or pieces of curved pipe. This picture is made up of seven different images from the Yohkoh spacecraft. It shows that loops come in many different shapes. Notice the little white bar on each image. It's about 43,500 km long, so you can see that loops come in many different sizes too. Some are much bigger than the white bar, and some are smaller. (The Earth is only about 12,750 km across, so most of these loops are much bigger than the Earth!) But what are the loops made of?
Sometimes the magnetic field comes up into the corona from the lower regions and loops back down into the Sun. As particles follow the path created by the magnetic field they form loops and arches that can be seen with special telescopes. The Yohkoh telescope that took these pictures is an X-ray telescope. The loops show up in X-rays because they are VERY hot (about 2,000,000 degrees Celsius!) and dense enough to show up in these X-ray pictures.
Image Credit: Yohkoh Science Team