The memories that Jonas has from childhood are true memories like the ones we have.
The ones that the Giver gives to Jonas are something more than what we know as memories. Somehow, it's like the Giver has these things on video or something. He actually "remembers" everything about some scene, even though he wasn't there.
By contrast, Jonas's own real memories are like ours -- they're sort of patchy and incomplete. When we think back about something that happened to us (first of all it does have to happen to us for us to remember) it's not like we have a perfectly clear memory with all the details -- we just remember bits and pieces, especially the things that really impacted us emotionally.
In the book "The Giver" Jonas has lived his life in an environment that was controlled by the elders. In essence his memories were chosen for him and shaped by the experiences that the elders allowed in the community. His memories were very much the same as the other children but with some differences. His acquired memories were the real things that Jonas had experienced.
The memories that came from the giver were a collection of memories that the giver had absorbed for the people. Jonas enters into a dreamlike state as he absorbs the memories through the transferring process. The memories feel like real experiences and change the way that Jonas thinks, but they are thoughts put into his mind not real life experiences in which Jonas had participated.