Ethereum

Plasma TLDR  

Plasma offers more security than a Validium but less than a full rollup

In a Validium if the operator disappears, the coins are permanently stuck

In a full rollup if the operator disappears, all users can exit their funds securely back to L1

In Plasma if the operator disappears, only users who deposited over a week ago will be able to exit their funds (there’s different ways to design it, this is just one example)

For L2s think of scalability/security as a spectrum: 0 is a Validium (lower security but highly scalable), and 1 is a full rollup (inherits Ethereum security)

There's also a middle ground: adding Plasma to a Validium boosts security significantly while staying scalable

/ethereum foundation | opinions are my own but more than likely they come from Vitalik's blog paragraph.xyz/@chaskin