To put it simply, EC2 security groups are for the particular EC2 instances which you have attached them to. But you can also attach the EC2 security groups to VPC.
On the other hand, a VPC security group can be only within the VPC. For example, if you have 2 VPCs under your account, the security group of the first VPC cannot be used in the second VPC. Under the VPC, you can apply the security group to any service (EC2, RDS).
If your account was created before 2013, then you would have come across EC2-classic and this is not available now. In EC2-classic, you don't need to launch the instances under a VPC so they had a separate security group. But if you are a new AWS user, then every resource should be launched within a VPC, thus making the security group a single entity.
Read this and get more insights on EC2 SG vs VPC SG.