Hong Kong Downtown to Guangzhou

Train
Coach

Train
There are 2 types of train.
Type1:One is through train which you get on the train station from Hunghom, Hong Kong, you pass the immigration counter in Hunghom and the train would arrive in Guangzhou directly. You do not need to get off in between to go through custom.
This is more expensive with ticket price HK$190 or 230, but more comfortable.? And generally, this is the most popular and common way for foreigner to travel from HK to Guangzhou.
It cost HK$190 or 230 per ticket, depends on if they would stop by Dongguan or the speed.
The fastest train can arrive in Guangzhou within 1 and 1/2 hr. and the one which stop by in Dongguan would take around 2 hr.
Generally, the ticket price of through train is double compare with the cross border coach from HK to Guangzhou.
The major advantage of through train is: you do not need to wait in line to go through immigration counter, custom clearance, as you go through the immigration counter in Hunghom, Hong Kong side and Guangzhou, China side, which they only cater to the passenger of the through train. So, you do not need to wait for a long time and it is more comfortable. Vs. the passenger of coach would go through say, Luo Ma Zhou or Huanggang, which lots of passenger would use that check point, which can be quite chaotic in peak hours.
Type2:The other is take the local train KCR train to Lohu, which is next to Shenzhen and then cross the bridge and walk to Shenzhen and then take the local train from Shenzhen to Guangzhou.
This would be much cheaper as it cost around HK$110 from HK to Guangzhou.? But you need to walk from Lohu to Shenzhen train station and wait for another train and you need to buy ticket from the ticket office in Shenzhen train station who cannot speak English. So, this is not highly recommended for traveler who is not in tight budget.

Through Train

Tele-ticketing and Enquiry Hotline : (852) 2947 7888
You can check the latest schedule from following linkage:
Website: http://www.it3.kcrc.com/B2C/UserPage/sysTimeTable_Eng.asp
You can reserve ticket online from following linkage:
e-Ticketing :http://www.it3.kcrc.com/b2c/frmIndex.asp?strLang=Eng
Usually, there is ticket available in weekdays, as this is more expensive than other means of transport, therefore, usually only business traveler would take the through train.?
But on weekdays or during Canton Fair period, it is necessary to reserve the ticket in advance.

Guangzhou -- Kowloon Through Train

Fare Chart
Guangzhou -- Kowloon Through Train

Coach

Take the coach from HK to Guangzhou is the cheapest and most common means of travel among general public and those who pay on their own.
Usually, it takes around 3 hrs. to travel from HK to Guangzhou, IF everything runs smoothly, no traffic congestion and no queue on the border.
Even though it seems the commuting time is double compare with through train, yet, since there is stop all over HK, so if you are not staying close to Hunghom station, or if you are not going to stay next to Guangzhou East station in Guangzhou, then it is very likely the door to door commuting time is similar to taking the through train.
Besides, if you are traveling with heavy luggage, which you need to choose either take taxi to Hung Hom train station or get on the coach from somewhere close to your hotel. Then you can save the taxi fare which usually cost over HK$100 from anywhere to train station ( as there is no metro station next to the train station) and the commuting and waiting time to train station by coach.
There are 2 types of cross border coach, one is those going to the destination directly, say, from HK to Guangzhou. The other is going to Huanggang, Shenzhen, i.e. border in China and you take another bus to your destination, say Guangzhou. These are called, cross border express bus.

Hong Kong to Guangzhou directly
In short, if you can confirm your schedule and there is a bus stop close to your hotel in HK, then take the direct bus would be the better option, as you do not need to look for another bus in Huanggang, Shenzhen.
Yet, as the schedule of those direct bus is less frequent, so maybe you cannot find the right bus at the time of your departure.
In general, the key pick up point of these cross border coach is Wanchai, on HK Island or Prince Edward in Kowloon.?
But there is many other pick up point all over Hong Kong .
You can check the detail from these sites:
http://ctsbus.hkcts.com/english/routes/dt-guangzhou.htm
http://www.cits.com.hk/2j/taxi/gj-1.jsp ( Pickup point, Mongkok and Shatin)? ( No English info )
http://www.trans-island.com.hk/eng/china-line.html#guangzhou
Pickup point, Prince Edward, Chaiwan, Taikoo Shing, North Point, Western District
http://www.gdhkmtc.com/schedule_e.html#a01
Pick up point :Sheung Shui, Shatin, Kowloon Tong, Honghom Stadium, ( No English info)
http://kongtung.com/kt/
Pickup point: Mongkok ( No English info)
http://www.wingleetravel.com.hk/new/enquiry7.htm
To Panyu, Shunde in Guangdong Province, which is close to Guangzhou ( No English info)?
http://www.cits.com.hk/2j/taxi/gj-1.jsp
Pickup point:? Mongkok, Shatin ( No English info)
http://www.eebus.com/ShowKowloonBusRoute.asp?linecode=G0013&name=?VOY?€
To Panyu, Shunde, Guangzhou
Pickup point: North Point, Western District, Lamtin, Kwuntong, Yaumatei, Tsimshatsui, Kowloon Bay, Shatin, Sheung Shui

Hong Kong to Guangzhou, taking a transition in Shenzhen

Other than taking the direct bus from HK to Guangzhou
You can also take those Cross Border Expresses, which commute from HK city to the border Huanggang.
EEBus runs the 24 hr. Kwun Tong, Yaumatei, Tsimshatsui Cross Border Express.
http://www.eebus.com/24h.asp

CTS runs the 24 hr. Mongkok, Wanchai and Kam Tin Cross Border Express
http://ctsbus.hkcts.com/english/routes/dt-allday.htm
After you cross the border in Huanggang, you can find the bus going to different part of Guangdong province.

As many of them can not speak good English, pls. print the file, so people can show you where to get on the bus.