It depends on the project. Many cryptocurrencies (Like Bitcoin, Etherium, Litecoin) have a public ledger. So every transaction (participants + amount) is visible.
Other Projects (where Monero is the most well known) use special cryptographic systems to hide all that. So the only thing which can be observed by 3th parties is that some transaction was successful somewhere. There are also projects which are private but prioritize efficiency (all the cryptographic hiding has a notable computation-cost) by sacrificing hidden amount or hidden participants.