Some typical bandwidth capacities for the common data transfer modes:

Mode Typical Maximum Rates
Dial up (using standard Copper Cabling) 56Kbps
Local Area Networking (ethernet cabling) 10/100 Mbps (for common setups)
ISDN 64 to 128 Kbps
Broadband (Cable & DSL) 512 Kbps to 1.5 Mbps
Fibre Optic Cables 100s of Mbps to Gbps
Wireless (Wi-fi) 802.11b 11 Mbps
Wireless (Wi-fi) 802.11g 54 to 108 Mbps (2.4 Ghz Frequency)
Wireless (Wi-fi) 802.11a 54 to 108 Mbps (5 Ghz Frequency)

Bytes v/s bits:

Most Internet service providers or ISPs and Webhosting companies will mention bandwidths in Kilobits per second. Whereas browsers and other file transfer agents show the speed in KiloBytes per second.

You may observe this and blame your ISP for falsely claiming high speeds. If you divide the KiloBits/second value by 8 (since 8 bits make up for a byte) and allow for some loss in transmission, you can get the true value of your actual transfer rate.

Hinderances to Data Transfer rates under normal conditions:
While advertised data transfer rates are based on laboratory calculations (similar to automobile manufacturers claiming X miles to a gallon, but you actually getting only X-y), the actual speed of transmission of data depends on one or more of the following.

a) Configuration of you PC (Processor, RAM, Number of simultaneous transfers taking place).
b) Packet Loss and Signal loss.
c) Server limitations.
d) Concurrent users requesting the server for data.
e) Hop counts and Latency.
f) Routing problems.