I’m confused about VLANS

I’m still somewhat of a newbie Network Admin, so excuse my ignorance. I’m not sure I understand the concept behind VLANS. For example, assume the Accounting department is on a separate VLAN from the Marketing department. However, both of them need access to the same servers. The only way, to my knowledge, for this to work would be for them to go through a router. Doesn’t this essentially negate the effectiveness of the VLANs? Isn’t there performance degradation associated with having to go through the router as opposed to just hitting the servers through a switch?

I’m sure I’m confused on multiple fronts, so any help would be appreciated.