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C++ is a powerful general-purpose programming language that is widely used for system/software development, game development, and more. It offers a fine blend of performance, efficiency, and abstraction. C++ is known for its strong emphasis on system-level programming, which provides the programmer with a lot of control over system resources and memory.

One of the key features of C++ is its support for procedural, object-oriented, and generic programming paradigms. This versatility makes it suitable for a wide range of applications. The syntax of C++ is based on C, which means that it will be familiar to anyone who has experience with C. However, C++ includes additional features such as classes and objects, exception handling, and templates which are not present in C.

Programming in C++ requires a good understanding of its syntax and features. Beginners should focus on learning how to write and compile programs using a mainstream integrated development environment (IDE) like Code::Blocks or Visual Studio, which often help in catching syntax errors and offer debugging tools.

In C++, input and output operations are handled using streams. A stream is an abstraction that represents a device on which input and output operations are performed. Therefore, when you want to display information to the user or read data from the keyboard, you will use streams.

Common streams include `cin` for input stream and `cout` for output stream, both of which are part of the C++ Standard Library. The `cin` stream is used to take input from the standard input device (usually the keyboard). It uses ‘>>’ operators to read data into variables. For example, `cin >> first;` reads an input and stores it into the variable `first`.

On the other hand, `cout` is used to send output to the standard output device (usually the screen). The ‘<<’ operator is used with `cout` to display data. For instance, `cout << "Hello, World!";` will display the text "Hello, World!" on the screen. Remember that each `cout` or `cin` operation must be terminated with a semicolon.

In C++, variables must be declared before they can be used. A variable declaration tells the compiler that a variable exists and reserves space in memory for it. The basic syntax for declaring a variable is to specify the type followed by the variable name; for example, `int temp;` declares an integer variable named `temp`.

C++ supports several basic data types like int, float, double, char, and more. Additionally, C++ provides support for user-defined types such as classes and structs. If a variable is declared without being defined, it can lead to errors when the program is run since the program will not know where to store the information.

In the context of our original exercise, the variables `first`, `last`, and `temperature` needed to be declared before use. This involved specifying the appropriate type, like `string` for names and `int` for numeric values. Always ensure you include the necessary headers (like `#include `) to support certain data types.

The semicolon is a critical part of the C++ language syntax. It is used to terminate statements, much like a period in a sentence. Without a semicolon at the end of a statement, the C++ compiler will not recognize where one statement ends and another begins, leading to syntax errors.

Each command or statement in C++ should be followed by a semicolon. For example, in `cout << "Hello, World!";`, the semicolon indicates the end of the command to display "Hello, World!". Forgetting to use a semicolon can make it difficult for the compiler to understand your code, resulting in errors when attempting to execute or compile the program.

In the exercise we corrected, semicolons were missing after some `cout` and `cin` statements. Adding these semicolons eliminated some syntax errors and made the program valid code that the compiler could process successfully. Paying attention to semicolon placement is essential for debugging and writing error-free code.