Question is really vague like others said. I'll just guess you're talking about "serverless" apps which are not completely without server :) Because as soon as something is online (and I guess you don't want to distribute it by post) it is on some server.
However, term "server" is wide. It can be a peace of rented hardware with OS (virtual or not, shared hosting or dedicated) or it can be a "service" like Amazon S3, Contentful, etc.
The later, recently is considered to NOT be a "server" but a service (although, server is behind it). That said, you can have a serverless app by mixing different services and technologies together like AWS S3 for static site hosting + Javascript/React for app code that fully runs on client side. In this case you only need static site hosting or some service like Netlify or AWS S3 to host it staticaly. Hell, you don't even need that, you can send it as a zip file via email in theory. So it runs fully on client, you only need Internet to download it.
Of course, without a database and connection to other online services (like email, facebook, etc) it will not do much (offline or online). So you can expand it with services like AWS Lambda + AWS API Gateway + AWS DynamoDB to build complex apps with dtabase and API backends, etc. It would be still considered "serverless" because you would base it on "services" and not "servers" but in practice, behind every service at the end there is a server :)