filled out basic concepts

Author: oleksandr-pavlyk

docs/docfiles/user_guides/manual/dpctl/basic_concepts.rst

@@ -34,16 +34,40 @@ more topical SYCL reference such as the `SYCL 2020 spec`_.
 
 * **Context**
    A context holds the run-time information needed to operate on a device or a
-   group of devices.
+   group of devices from the same platform. Contexts are relatively expensive
+   to create and should be reused as much as possible.
 
 * **Queue**
-   A queue is needed to execute any computation on a device.
+   A queue is needed to schedule executition of any computation, or data
+   copying on the device. Queue construction requires specifying a device
+   and a context targeting that device as well as additional properties,
+   such as whether it collects profiling information or whether submitted
+   tasks are executed in the order in which they were submitted.
 
 * **Event**
-   *Needs updating*
+   An event holds information related to computation/data movement operation
+   scheduled for execution on a queue, such as its execution status as well
+   as profiling information if the queue the task was submitted to allowed
+   for collection of such information. Events can be used to specify task
+   dependencies as well as to synchronize host and devices.
 
 * **USM**
-   *Needs updating*
+   Unified Shared Memory (USM) refers to pointer based device memory management.
+   USM allocations are bound to context. In other words, a pointer representing
+   USM allocation can be unambiguously mapped to the data it represents only
+   if the associated context is known. USM allocations are accessible by
+   computational kernels that are executed on a device, provided that the allocation
+   is bound to the same context that was used to construct the queue where the
+   kernel was scheduled for execution.
+
+   Depending on the capability of the device, USM allocations can be a "device"
+   allocation, a "shared" allocation, or a "host" allocation. A "device" allocation
+   is not accessible from host, while "shared" or "host" allocations are. "Host"
+   allocation refers to an allocation in host memory that is accessible from a device.
+
+   "Shared" allocations are accessible by both host and device. Runtime manages
+   synchronization of host's and device's view into shared allocations. Initial
+   placement of the shared allocations is not defined.
 
 .. todo::