802.11 networks come in two main flavors. "Managed" or "infrastructure" networks (that Kismac identifies as <managed>) involve one or more dedicated "access points". In an "ad hoc" network (Kismac: <ad hoc>), individual computers communicate directly, without any access point involved.
Kismac may show <hidden ssid>, <no ssid> or blank when it hasn't yet detected an SSID. I'm not precisely sure when each of those is shown (and mileage has varied between different versions of Kismac).
A "probe request frame" is sent by computers trying to find access points. Asking for access points with a specific SSID (or via "broadcast SSID" to try to get a response from any access point within range). Access points send "probe response frames" back. (However, hidden access points will generally not respond to probes to "broadcast SSID".) The Kismac <probe> shows probes.