Common Language Runtime :
CLR is the core engine of .Net framework.
CLR provide the following services :
1. Memory management
2. Thread management
3. Exception handling
4. Garbage collection
What is JIT ?
The Common Language Runtime (CLR) provides various Just In Time compilers (JIT) and each works on a different architecture depending on Operating System. That is why the same Microsoft Intermediate Language (MSIL) can be executed on different Operating Systems without rewrite the source code. Just In Time (JIT) compilation preserves memory and save time during application initialization. Just In Time (JIT) compilation is used to run at high speed, after an initial phase of slow interpretation. Just In Time Compiler (JIT) code generally offers far better performance than interpreters.
CLS (Common Language Specification):
CLS is a standard for .net . cls is small set of specification to make all languages as a .net compliant languages. cls make a use of cts and clr. if my languages (c#,vb.net,j#,vc++) wants to be compliant language it has to follow cls standard.
CTS (Common Type System):
Common Type System is also a standard like cls. If two languages (c# or vb.net or j# or vc++) wants to communicate with each other, they have to convert into some common type (i.e in clr common language runtime). In c# we use int which is converted to Int32 of CLR to communicate with vb.net which uses Integer or vice versa