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The major difference between threads and processes is:

1 Threads share the address space of the process that created it; processes have their own address space.
2 Threads have direct access to the data segment of its process; processes have their own copy of the data segment of the parent process.
3 Threads can directly communicate with other threads of its process; processes must use interprocess communication to communicate with sibling processes.
4 Threads have almost no overhead; processes have considerable overhead.
5 New threads are easily created; new processes require duplication of the parent process.
6 Threads can exercise considerable control over threads of the same process; processes can only exercise control over child processes.
7 Changes to the main thread (cancellation, priority change, etc.) may affect the behavior of the other threads of the process; changes to the parent process does not affect child processes.

why can you run multiple threads but not multiple processes?

On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time yielding an illusion of concurrency.

thread is a lightweight process whereas process is job under execution.

Just as u have multiple threads within a process, similarly u have multiple process within a user/kernel space.
We can have multiple process too!