TCP is backed by acks and retries to make sure you data gets where it's going. UDP is connectionless and "fire and forget". UDP is mostly used for streaming type applications, where if you lose some data you don't need to try to send it again.
Which one you use depends on the application. For example, a web server uses TCP.
Unmanaged switches — These switches have no configuration interface or options. They are plug-and-play. They are typically the least expensive switches, found in home, SOHO, or small businesses. They can be desktop or rack mounted.
Managed switches — These switches have one or more ways, or interfaces, to modify the operation of the switch. Common management methods include: a serial console or Command Line Interface accessed via telnet or Secure Shell; an embedded Simple Network Management Protocol SNMP agent allowing management from a remote console or management station; a web interface for management from a web browser. Examples of configuration changes that one can do from a managed switch include: enable features such as Spanning Tree Protocol; set port speed; create or modify VLANs, etc.
Two sub-classes of managed switches are marketed today:
Smart (or intelligent) switches — These are managed switches with a limited set of management features. Likewise "web-managed" switches are switches which fall in a market niche between unmanaged and managed. For a price much lower than a fully managed switch they provide a web interface (and usually no CLI access) and allow configuration of basic settings, such as VLANs, port-speed and duplex.[10]
Enterprise Managed (or fully managed) switches - These have a full set of management features, including Command Line Interface, SNMP agent, and web interface. They may have additional features to manipulate configurations, such as the ability to display, modify, backup and restore configurations. Compared with smart switches, enterprise switches have more features that can be customized or optimized, and are generally more expensive than "smart" switches. Enterprise switches are typically found in networks with larger number of switches and connections, where centralized management is a significant savings in administrative time and effort. A Stackable switch is a version of enterprise-managed switch.
Source: http://en.wikipedia.org/wiki/Network_switch
I would explain in more personal detail, but the wiki explains it pretty well.
Best Answer
Sounds like you're talking about Windows, eh?
PING sends ICMP echo request datagrams to the destination host specified. The destination host will send back ICMP echo reply datagrams, if it is so configured. People typically think of using PING to judge the "reachability" of a host. You can vary the size of PING packets and watch the round-trip time to learn a little about the network bandwidth conditions (as well as, obviously, measuring latency) between the source and destination hosts.
A rookie mistake, BTW, is thinking that because you never receive a reply your echo request datagram isn't getting to the remote host. The absense of replies tells you that your echo request didn't reach the destination host OR the reply from the destination host didn't make it back to you. The absense of PING replies doesn't tell you anything conclusive.
The Microsoft TRACERT utility seeks to provide similiar functionality to the Unix "traceroute" utility. Microsoft's TRACERT sends out ICMP echo request datagrams with increasing "time to live" (TTL) values. The TTL determines how many hosts an IP datagram can be forwarded through. Each host that forwards the datagram decrements the TTL until it reaches zero, at which point the host that decremended the count to zero sends an ICMP time to live exceeded datagram to the host that sourced the datagram that has ceased to "live". By sending out datagrams with increasing TTL values and watching where the time to live exceeded datagrams come from one can produce a "map" of the "hops" that the outbound datagram moved through, along information about how long it took for the datagram to round-trip.
Incidently, the Unix "traceroute" uses UDP datagrams rather than ICMP to perform a similar function.