There are two key variables to consider when choosing Cello strings; core type and gauge.
For hundreds of years, strings could only be manufactured with gut core, made from sheep intestine. Gut core cello strings have the warmest sound, complex and rich with overtones and are used primarily by professional classical musicians. These strings can be difficult to deal with in terms of over-sensitivity to temperature and taking a long time to settle in. With modern technology, synthetic core strings are now available, which, as closely as possible, replicate the tone and feel of the gut core string, while avoiding some of its disadvantages. The other option is a a steel core string, which, like a guitar string, is comprised of a straight or twisted wire, wound with metal. These are the most stable and durable and offer stable pitch, high volume, but less overtones.
The most commonly made cello string gauges are light, medium and heavy , but many manufacturers offer variations using gauge numbers. Generally speaking, heavier strings produce more tone, but have a slower response and lighter strings have a fast response but thinner, lower volume sound.