In OAuth 1, there are two components to the access token, a public and private string. The private string is used when signing the request, and never sent across the wire.
The most common way of accessing OAuth 2.0 APIs is using a “Bearer Token”. This is a single string which acts as the authentication of the API request, sent in an HTTP “Authorization” header. The string is meaningless to clients using it, and may be of varying lengths.
Bearer tokens are a much simpler way of making API requests, since they don’t require cryptographic signing of each request. The tradeoff is that all API requests must be made over an HTTPS connection, since the request contains a plaintext token that could be used by anyone if it were intercepted. The advantage is that it doesn’t require complex libraries to make requests and is much simpler for both clients and servers to implement.
The downside to Bearer tokens is that there is nothing preventing other apps from using a Bearer token if it can get access to it. This is a common criticism of OAuth 2.0, although most providers only use Bearer tokens anyway. Under normal circumstances, when applications properly protect the access tokens under their control, this is not a problem, although technically it is less secure. If your service requires a more secure approach, you can a different access token type that may meet your security requirements.