Being new to VS, how may I think of these two concepts, what is the difference?
A solution is a container for projects, and tracks dependencies between projects.
I find some missing information in the other answers (at least for people who come from other IDEs like, say, Eclipse) . To say that a solution is a container for projects is only part of the thing. The conceptual feature of a VS project (what determines its 'granularity') is that one project produces one output: typically an executable or a library (dll). So, if you are going to code three executables that uses related code, you'll create one solution and at least three projects - probably more.
Just to come up with a metaphor..
A solution is like a house, a project like a room. Each room provides a piece of functionality whereas the house, a container of rooms, provides the means to link the rooms together and organize them appropriately.
Kind of corny but I made it up on the fly, so bear with me :)
It doesn't help that Visual Studio seems to make things more confusing. "New Project" actually creates a new SOLUTION containing a project. "Open Project" actually opens a solution containing one (or many) project. (The file menu says "Open Project/Solution" but it really is opening solutions. There is no "Close Project" only "Close Solution" which is accurate.
So, in VS you are always working within a solution. Many solutions contain only one project and newer developers are likely to think of them as the same thing. However you can add other projects into a solution.
In case anyone decides to scroll down this far... I thought the MS docs did a pretty good job at describing the differences. I've copy pasted (and rephrased) the relevant bits here:
When you create an app, application, website, Web App, script, plug-in, etc in Visual Studio, you start with a project. In a logical sense, a project contains of all the source code files, icons, images, data files and anything else that will be compiled into an executable program or web site, or else is needed in order to perform the compilation. A project also contains all the compiler settings and other configuration files that might be needed by various services or components that your program will communicate with.
You don't have to use solutions or projects if you don't want to. You can simply open the files in Visual Studio and start editing your code.
In a literal sense, a project is an XML file (
.vbproj,.csproj,.vcxproj) that defines a virtual folder hierarchy along with paths to all the items it "contains" and all the build settings.In Visual Studio, the project file is used by Solution Explorer to display the project contents and settings. When you compile your project, the MSBuild engine consumes the project file to create the executable. You can also customize projects to product other kinds of output.
A project is contained, in a logical sense and in the file system, within a solution, which may contain one or more projects, along with build information, Visual Studio window settings, and any miscellaneous files that aren't associated with any project. In a literal sense, the solution is a text file with its own unique format; it is generally not intended to be edited by hand.
A solution has an associated .suo file that stores settings, preferences and configuration information for each user that has worked on the project.
A Solution can have many Projects.
The Solution can also handle managing the dependencies between its different Projects...making sure that each Project gets Built in the appropriate order for the final Solution to work.
A project contains executable and library files that make up an application or component of an application.
A solution is a placeholder for logically related projects that make up an application. For example, you could have separate projects for your application's GUI, database access layer, and so on. The projects would be specific divisions for your program's functionality, and the solution would be the umbrella unifying all of them under one application.
Solutions are containers for projects - you can also use them to organize items that are used across different related project (shared dll's and such).
A solution is a readable text file whose extension is .sln and having a structured content that describes the projects that it contains. A project is a readable XML formatted text file whose extension is .vcxproj and having a structured content according to its XML schema, and whose primary purpose is to contain the listing of source codes filenames and their dependencies or references to other project's source codes as well.
Solutions are containers used by Visual Studio to organize one or more related projects. When you open a solution in Visual Studio, it will automatically load all the projects it contains.
When you create a new project in Visual Studio, it automatically creates a solution to house the project if there's not a solution already open.
You can set dependencies of projects on other projects in the solution. The dependent project is build after the project it is depending on is built.
For more details refer - https://docs.microsoft.com/en-us/visualstudio/ide/quickstart-projects-solutions
If you are from an Eclipse background you would probably go to build path of a project and add a dependency on other project or add an external jar. In VS you can do that in a single container called solution where all related projects are grouped together.
Eg. Let's say you are build and android and iOS app in xamrin, there would be some common code and resources that could go in a separate project and then your android and iOS projects can depend on this common code project. Also you could have projects to test these projects etc.
来源：https://stackoverflow.com/questions/2389226/visual-studio-project-vs-solution