Running tops come in different variations and like all clothing will be influenced by the time of year.
Generally in the Summer, runners will wear either a short sleeved t-shirt like the Nike Men’s Race Day Short Sleeve Tee or a singlet like Nike Women’s Swift Racer Long Top due to the temperature. Running tees and singlets will be made from technical fabrics designed to reduce friction with minimal seams, as well as offer UV protection from the Sun and perhaps most importantly wick the sweat away from the skin, keeping you drier and more comfortable. They also tend to have extra ventilated areas where needed and some will have a small pocket for keys, energy gels or anything else small. Don't underestimate the importance of a good running top in the summer as it's not nice running with a soaking wet ordinary t-shirt that's sticking to you and weighing you down.
In the Autumn to Winter months depending on how cold it is you can get away with wearing just a long sleeved top. These can vary in thickness, the thinner ones can even be worn in the spring and summer months. Long sleeved tops come as a v-neck, crew neck or even a half-zip. They will also be made of technical fabrics designed to wick the sweat away from the skin but also offering extra warmth than a short-sleeved tee. When it's cold they are perfect to wear underneath a running jacket.
Base layers are designed to be lightweight and comfortable but most importantly to regulate your temperature whether its hot or really cold. Base layers are generally worn underneath other clothing like another short or long sleeved top or a jacket. They are tight fitting, offering extra ventilation at the key areas and is a must for runners looking to tackle the cold. Base layers are used across all sports from football to skiing as its a superbly functional garment. A runner should need no more than a base layer underneath a running top with a running jacket. This combination is lighweight and prepares a runner for the coldest, windiest and wettest conditions.
Compression tops are a type of base layer with the same properties but with the added benefit of compression. Compression aids performance and recovery by increasing blood circulation thereby providing more oxygen to the muscles. This makes a big difference when your training starts to become intense with fewer rest days, allowing you to recover quicker and reducing DOMS (Delayed Onset Muscle Soreness). Leading brands like Skins and 2XU are used across a wide range of sports and are particularly popular amongst Triathletes due to their rigorous training.