Difference between MPI_Allgather and MPI_Alltoall functions?

What is the main difference betweeen the MPI_Allgather and MPI_Alltoall functions in MPI?

I mean can some one give me examples where MPI_Allgather will be helpful and MPI_Alltoall will not? and vice versa.

I am not able to understand the main difference? It looks like in both the cases all the processes sends send_cnt elements to every other process participating in the communicator and receives them?

Thank You