A free SMS service is normally run through a website. It usually simply involves typing your message and the recipient's number on the site and clicking a send button. These services are open to misuse by spammers, but can be useful for people on a budget, or those who have run out of credit on a prepaid phone.
Like any free service, you should always remember to check how the company makes its money. In some cases, it will simply involve the sender seeing advertisements, which can be acceptable if they aren't intrusive. In other cases, the person receiving the message will also see an advertisement. This may be irritating to them and could be a particularly bad idea if you are sending business-related messages.
Another point to check with a free SMS service is how your number is displayed to the recipient. The best services will allow you to link an account to your own number so that it is displayed as the sender. Others will either include your number in the message or expect you to include it yourself. These can be problematic as the recipient will not be able to simply hit the reply button on their phone and the message may even appear to be mobile spam.
Remember that the best cheap SMS service isn't necessarily one that is free. It's important to check the reliability of services, for example, by searching for online reviews. Using a free SMS service from a company with a patchy reputation for actually delivering the messages speedily may be a false economy.
Before searching for a free SMS service, it's worth checking that there aren't better value options with your current cellphone provider. If you send a lot of texts, it may be worth switching to a more expensive monthly plan that allows you a certain number of texts each month, or even an unlimited number of texts. It's also worth finding out what networks the people you text most often are on. Some networks allow you cheaper or free texts to others on the same network, to the point that changing network may even work out the best value.
Another option is Voice Over Internet Protocol services. These are mainly designed for making phone calls over the Internet, which can be much cheaper with long-distance or international calls. Some of these services offer cut-price texts, plus you have the benefit of being able to type on your keyboard rather than your phone. The main disadvantage is that such services normally require you to pay a lump sum up front to get credit.