In patent classification systems number is assigned to a subject. This can be very helpful when searching patent databases, as there can be many different words to describe the same topic. If you can find a number for your topic you don't have to search for all the possible words, you can use the number instead.
IPC is managed by the World Intellectual Property Organisation (WIPO). The main categories are:
A - Human necessities
B - Performing operations; transporting
C - Chemistry; metallurgy
D - Textiles; paper
E - Fixed constructions
F - Mechanical engineering; lighting; heating; weapons; blasting
G - Physics
H - Electricity
It is a hierarchical system and you can drill down to a very specific subject. For example:
G Physics
G02 Optics
G02C Spectacles, sunglasses or goggles ...
G02C5 Construction of non-optical parts
G02C5/14 . Side members
G02C5/20 .. Adjustable, telescopic
This enables you to browse for the IPC code for your chosen topic but you cannot find patents here. To search for an IPC code, use the Derwent Innovations Index.
The Cooperative Patent Classification (CPC) is based on the IPC but gives a more specific level of detail. It is jointly managed by the EPO and the US Patent and Trademark Office. Sections A-H are organised in the same way but there is an additional section:
Y - General tagging of new technological developments; etc.
You can find the CPC code for your chosen topic here and also search for the code in the Espacenet database.