"Ophalen" implies a prior arrangement in the sense of the English verb "to collect", i.e. that something or someone is waiting/ready to be picked up: 1. Ik moet mijn nieuwe rijbewijs ophalen bij de gemeente. – I have to pick up my new driver's license at the municipal office. 2. Wanneer ging jij de kinderen ophalen? – When did you want to pick up the kids? "Halen" means "to get" or "to pick up" in a more general sense: 1. Ik ga nog even een flesje water halen. – I'm just going to get a bottle of water. 2. Zullen we vanavond een pizza halen? – Should we get a pizza tonight? "Halen" can sometimes be used in the sense of "ophalen" ("Kom je mij halen?" – "Are you coming to pick me up?"), but it doesn't work the other way round ("Ik ga een flesje water ophalen" is wrong).