What exactly is the difference between an access port and a trunk port?

The difference is tagging. An access port belongs to ONE VLAN and carries plain, untagged Ethernet — the endpoint has no idea VLANs exist. A trunk carries many VLANs at once, and every frame gets an 802.1Q tag inserted (4 bytes carrying the VLAN ID) so the far end knows which VLAN each frame belongs to. Rules of thumb: switch→PC/printer/server = access (switchport mode access, switchport access vlan 10); switch→switch or switch→router = trunk (switchport mode trunk). The subtlety is the native VLAN: one VLAN per trunk travels untagged (default VLAN 1). If the native VLANs differ on the two ends, frames silently cross VLANs — match them, and preferably move native off VLAN 1.