Is Acer Aspire V 5 571 G an Ultrabook ? Because it is very thin with DVD player.
The difference is that i3 is mostly lower clocked and doesn't have boost. i5 usually has higher clockspeeds and does have boost. Boost is when the CPU goes to higher clockspeeds when under load.
This is only with notebooks. As it is a laptop you're asking about.
On desktops i3 only got 2 cores, but 4 threads, still no boost. On desktops i5 got 4 cores, 4 threads, with boost.