Cleos Endpoint Usage 
In many cases when working with smart contracts, or on-chain actions you will need an endpoint. An endpoint can be described as a URL to push transactions to, get chain information such as tables, or lookup accounts.
Cleos Usage 
Cleos has a section on using external endpoints.
In short append the -u flag with a url. If you are using local environment then this flag can be skipped. The default value of http://127.0.0.1:8888 will be used
cleos -u <endpoint>cUrl Usage 
cUrl can be used normally but suggested to use json_pp to help prettify the output from a cUrl response.
curl -X POST <endpoint>/v1/chain/get_infowget Usage 
wget can also be used to fetch endpoint information as well.
wget -qO- /dev/stdout <endpoint>/v1/chain/get_info | json_ppAvailable Endpoints 
See this API section for information on block producer endpoints that are publicly available.
Limitations and Availability 
Each of these endpoints has their own policies and limitations on data that can be sent to and pull from the endpoint. However, those limitations are not easily understood. These are all public facing endpoints and if an API node is down, simply try another one.