In simple words, IoT is combination of all engineering solutions useful to all industries. Just to take few examples:
Automotive industry: it is being aggressively used in modern vehicles to monitor, track, control, and even lock/unlock the vehicles. Review Tesla cars to get a wide range of applications.
Home automation and security: Latest home security systems are being used for monitoring and unusual activity notification to remote devices.
Agriculture industry: Now government is getting latest and accurate information about the weather condition, fields condition, crop condition, etc. to provide help to farmers at earliest.
Medical industry: Latest gadgets not only keep track of your health but also enabled to notify remote systems or persons to cater to medical needs.
The list will keep on growing and growing, as many as we can write Same is the situation with the use of IoT. you just imagine to integrate, sense, apply some logic, take some action with help of any mechanical, electrical, electronic, or any device which can connect electronically to any system. Simply said, there is no limit, even sky is not the limit, as we use IoT in space too, as we were able to track the progress of spacecraft went to moon. Yes, IoT is even used in chandrayaan 2!
So how it works?
Following are basic components of an IoT solution:
Input device - Data sensor like RFID, Door sensor, Bar code, QR code, heat sensor, camera sensor, proximity sensor, bluetooth, etc. These will sense the data and send it to central processing unit.
IoT Controller - It is central processing unit which receives the data, analyst the data and then take different actions. The actions may be storing data in a database, applying logic to define next step. send the output single to next device to take action. It is done with help of a programming language.
Output device - these are devices like mobile for notification, electric switch for any action, email, database update script etc.
In center of all components we have internet as a medium to transfer the information. there are different protocols being used.
Now you should be able to understand why everyone says, there is no limit of IoT solutions. Your imagination and your creativity are your best friend to create any innovative solution. You may take the guidance to know about it at IoT guidance page.