what is the difference between qsub and ./

Can anyone tell me the difference between the following ways of submitting a script: $ qsub script_name.sh and ./script_name.sh What are the differences between the above two ways of submitting a job to a cluster? Also how come sometimes I need to type: $ chmod +x script_name.sh ...before I can type ./script_name.sh to submit a job? How come sometimes I just need to type qsub script_name.sh? Sorry I'm not very familiar with Unix.