The Celeron processor is Intel's lowest category - it will be much less powerful than an i3 or i5. That said, it has enough power to browse the web, use office applications, and watch HD video.
A Core i processor is significantly more powerful than a Celeron processor. It's worth noting that upgrading your CPU, even if you only use your laptop for simple chores, will ensure that your laptop is capable for a longer amount of time.