The answer to 'difference between weed control' is that there are two main approaches to weed control: pre-emergent and post-emergent. Pre-emergent weed control products are applied before weeds germinate, preventing them from establishing in the first place. Post-emergent products are used to kill existing weeds after they have already sprouted.
Pre-emergent weed control is generally more effective at preventing a weed problem, but requires more planning and timing to apply at the right time of year. Post-emergent products can be used to spot-treat weeds as they appear, but may need to be reapplied more frequently. The best approach depends on your specific landscaping needs, budget, and level of DIY maintenance.