Difference Between Paid & Free VPN?

I always wonder what's the difference between paid and free VPNs.

The most readily possible reason I can think of is the no. of servers/routers between us and the website. For all we know paid vpn service may just be: me <--> Paid VPN server <--> website.

I'm don't have a technical background and wonder how can we tell how secure a particular VPN service is?

I hope someone here can enlighten me.

Thanks a lot (in advance)!